[TanksAndSpartans]

This is actually an interesting academic question. Economics 101 would certainly tell you its completely irrational to behave differently because in one case the amount you pay is what you bid (plus shipping) and in the other case the amount you pay is what you bid + 20% + shipping. The rational thing to do is make the calculation of adding the BP and behave (bid) the same way in both cases (i.e. reduce your bid because you know you will be hit with a BP at checkout). Someone would need to do a study to see if people actually behave according to the theory or whether the fact that that 20% in not included in their bid induces them to pay more as if they are willing to momentarily forget they will be charged the 20% later. It's entirely possible, and also the reason I believe some auction houses want you to have to do the math in your head (or not do it), rather than show you the BP when you place your bid. I think its something called the framing effect where people view options that are really the same as different depending on how they are stated.

One minor point is because the bid increments are wider for AHs than eBay, sometimes I have to make a decision whether I want to go under or over my target - something I never really have to do on eBay where the bid increments are very narrow.