I don't think it should be allowed to be advertised on a post. Using the gift option for a sale is using a service without paying for it, period. I don't believe a website should endorse or allow what is in a sense thievery. Not to mention, it's there for a reason... it protects both the buyer and seller. All such fees should be included in whatever price is listed. On shipping, while I prefer it being included in the price, I can understand separating it due to varied costs and multiple items being shipped together.
If people have a trusting history want to work out a "discount" or whatever for using a gift option behind closed doors, they can go right ahead. Much like if a person makes a living selling knives and doesn't pay taxes on the income, that wouldn't be the site's issue at that point.
Besides, I know when I see "$$$ +3%," I get a sense that the seller is someone who nickle and dimes for everything... I don't like dealing with people that nickle and dime for everything.