Buying boxes from a seller that has a set size range may be good in some ways if your product happens to fit the box. However many Artists and Craftspeople make and create items of varying sizes. Having a box the correct size can save packaging cost, provide better protection, save void fills particularly plastic bubble for example and generally give you customer a better opening experience.
Packaging for online sales is all part of your offering. Get it wrong and buyers will remember that. Get it right and they’ll return for more and tell others about it..