Not sure if this is the right forum - I'm sure 20 angry posters, and an angry mod, will tell me if it's not 
I've done a bunch of trades and purchases recently, and I'm actually kind of surprised how many people manually write out shipping addresses, buy shipping at the post office counter, etc. If that works for you - no problem. I mentioned this to a few people I know who trade a lot, and they said going to the PO is easier for them.
For everyone else, though, I figured I'd add some thoughts on how to make shipping easier, and hopefully more secure.
USPS Priority Shipping Materials:
You can get free USPS Priority flat rate shipping boxes and envelopes at the post office, or you can order them online at USPS.com. If you order online, the materials are delivered for free to your front door.
This includes Priority Mail Flat Rate Padded Envelopes. That's not the cheap cardboard envelopes - these are Tyvek with a bubble wrap lined interior. You can mail in them directly using the Padded Flat Rate Envelope rate, or use them as padding inside a larger box. In fact, a Small Flat Rate Box will fit inside the padded envelope, for additional protection.
USPS Priority Flat Rates:
I've gotten a lot of small knives sent in random boxes with USPS Priority generic rates. If you use a Small Flat Rate Box, Flat Rate Envelope, or Padded Flat Rate Envelope, the flat rate will be as cheap (or cheaper) than even the lowest rate you'd get using a generic box. So, flat rate will save you money even on the smallest shipments.
Printing Labels Online:
Similarly, I've seen people manually write out the shipping address, and buy postage at the PO. Assuming you have a printer in your home or office, I think it's easier to just print at home. Several options:
That'll also save you time waiting in line at the post office. For secure drop-off, most POs I've been to have mail drop slots inside the building (near the PO boxes section, so open 24/7) that will fit small flat rate boxes and envelopes. One PO near me has a giant drop slot open 24/7 that'll fit very large boxes.
Finally, my handwriting is horrible, so printing online saves me from the risk of having a package misdirected if the delivery guy can't read my handwriting - just another layer of security
Other Thoughts:
The adhesives used on the free shipping material is underwhelming. I always add extra tape to keep it safe - Scotch Heavy Duty Packing Tape is great stuff, and great insurance when shipping around expensive knives.
I also typically print two shipping labels - one for the outside of the box, and one that I place next to the knife itself. While it's never happened to me, I've heard of boxes getting damaged where the shipping label becomes unreadable, and the postal service needs to figure out where to forward the contents. Most people don't include packing slips or other labels inside, in which case your beautiful knives will end up at an undeliverable mail auction. Adding a second label packed with the knife would help prevent this.

I've done a bunch of trades and purchases recently, and I'm actually kind of surprised how many people manually write out shipping addresses, buy shipping at the post office counter, etc. If that works for you - no problem. I mentioned this to a few people I know who trade a lot, and they said going to the PO is easier for them.
For everyone else, though, I figured I'd add some thoughts on how to make shipping easier, and hopefully more secure.
USPS Priority Shipping Materials:
You can get free USPS Priority flat rate shipping boxes and envelopes at the post office, or you can order them online at USPS.com. If you order online, the materials are delivered for free to your front door.
This includes Priority Mail Flat Rate Padded Envelopes. That's not the cheap cardboard envelopes - these are Tyvek with a bubble wrap lined interior. You can mail in them directly using the Padded Flat Rate Envelope rate, or use them as padding inside a larger box. In fact, a Small Flat Rate Box will fit inside the padded envelope, for additional protection.
USPS Priority Flat Rates:
I've gotten a lot of small knives sent in random boxes with USPS Priority generic rates. If you use a Small Flat Rate Box, Flat Rate Envelope, or Padded Flat Rate Envelope, the flat rate will be as cheap (or cheaper) than even the lowest rate you'd get using a generic box. So, flat rate will save you money even on the smallest shipments.
Printing Labels Online:
Similarly, I've seen people manually write out the shipping address, and buy postage at the PO. Assuming you have a printer in your home or office, I think it's easier to just print at home. Several options:
- If your customer bought via PayPal G&S, you can buy shipping directly from the PayPal transaction
- You can also buy shipping from PayPal by searching for "multi order shipping" in the search bar. That will let you manually enter an address
- Finally, you can buy shipping from USPS.com (Menu > Mail & Ship > Tools > Click-n-Ship). They accept credit cards or PayPal
That'll also save you time waiting in line at the post office. For secure drop-off, most POs I've been to have mail drop slots inside the building (near the PO boxes section, so open 24/7) that will fit small flat rate boxes and envelopes. One PO near me has a giant drop slot open 24/7 that'll fit very large boxes.
Finally, my handwriting is horrible, so printing online saves me from the risk of having a package misdirected if the delivery guy can't read my handwriting - just another layer of security

Other Thoughts:
The adhesives used on the free shipping material is underwhelming. I always add extra tape to keep it safe - Scotch Heavy Duty Packing Tape is great stuff, and great insurance when shipping around expensive knives.
I also typically print two shipping labels - one for the outside of the box, and one that I place next to the knife itself. While it's never happened to me, I've heard of boxes getting damaged where the shipping label becomes unreadable, and the postal service needs to figure out where to forward the contents. Most people don't include packing slips or other labels inside, in which case your beautiful knives will end up at an undeliverable mail auction. Adding a second label packed with the knife would help prevent this.