Bases supplied by mail order may be supplied bulk packed, rather than in separate bags. Note that all bases are listed by the larger dimension first. Being laser cut, the bases have a layer of dark brown or black "soot" on the edges. The soot will wipe off though the edge will remain black.
Depending on your base embellishment techniques the largest pieces of MDF may require sealing on both sides with dilute PVA before use so as to reduce any possible warping. This shouldn't be an issue with most sizes.