Hi Lycosa,
A long time ago a knife maker told me "The toughest knife to make is the one that has already been paid for!"
NEVER EVER PAY A KNIFE MAKER IN FULL IN ADVANCE. If you want to know why...search The Good Bad and Ugly. You will find story after story of collectors who did just that and either it was months, years or they never received the knife.
Now, that being said. Are you asking for some unusual knife? High end handle material, gold, jewels,etc. If yes, then it is fair for the maker to ask you to pay for these materials up front.
What your makers should do...and this is for all makers. Is contact you prior to starting your knife. Go over the order again so there is no confusion. Discuss payment at that time.
With the understanding that if the maker is more than 2 weeks late on his delivery date..it is your option to get your money back in full.
Also, that if you are more than two weeks late in paying...it is the makers prerogative to sell the knife.