I believe you are correct in general, BigBox seems to have ~20% discount whereas Boards&Bits, Cool Stuff, & Mini Market are usually in the 30% to 40% range.
However, I am looking to buy some old Magic Theme Decks which seem to be sold out or at a premium price everywhere else ($8 at BigBox vs $24 on Amazon). So I just wanted to make sure that they were a reliable operation.
OK. I have never dealt with them. Sorry I can't help.