>> How big of a purchase is that compared to what the store usually sees, <<

Much, much bigger. Most of the time people come into craft stores to buy one or a few things. Serious mavens can rack up $50-100.

>> and how often does it see a purchase on that scale? <<

Probably once or twice a month on average, when a teacher comes in for a major stockup. Numbers would skew upward in August (back to school) and November (holiday shopping) though.

>> Would the store's record's show how payment was made, or just the amount? <<

Just the amount if it's cash or cash equivalents like Greenbucks. Credit cards, checks, etc. leave a trail.