I would think outside the box and have them make... say, mouse pads advertising AG's most popular. I would buy a Bubbletanks one for $20 (exaggeration) definitely =)
We are talking about additions to the shop, right? Or just the part about buying AP with cash? It's like the same thing with all other gaming sites, like WoW, buying virtual gold, so you wouldn't have to go through all the effort getting it legitimately. Then there are those MMOs where you have to pay to get the full value of it. Even if AG is a sort of game site, I wouldn't want it to be getting money like that. Merchandise is better.
Ah, like incentive. But this site is just not that kind of thing. MMO's and all those sites with better money do, but this is just one more site, like miniclip, newgrounds etc. I fail to say this idea wouldnt fail, but I like your pluck.