I'd recommend going for the full package (with GBA expansion) from the start - then the expansion only adds about $10 to the price, instead of $20+
when you factor in a second shipping cost for the second order, you'd be saving at least $14, which is nearly the cost of the expansion. And it is worth it - in addition to some fantastic GBA games out there, you'll also find some homebrew that just won't work without some extra memory (Quake2DS anyone?). I didn't think I'd need it at first either, but mine was ordered within a week of receiving my M3, and my only regret since is that I basically paid more than twice what I could have ($10 vs $26).