The way this is supposed to work is that they type in a little number on the cash register, you pay, and they print you out a receipt. I opened up the email, wrote down the big number at the top, and walked down to the 7-11...in the rain. (Yes, it would choose today to rain.)
The cashier tried to type the number in, but it wouldn't take. The register just kept beeping at her. There was nothing for me to do but walk back to work.
I checked the email again, and this time I scrolled down to the very bottom to find a much smaller "payment number." It seems the number I had written down was Amazon's order number. Oh, well. I printed out the entire page (just so I could show it to the cashier, so she could read it and know I'm not crazy, just dense) and took it back to the store.
It still didn't work.
I walk back to school again. Still in the rain.
I open up the email again and input the web address they listed for questions about payment. After clicking through several links, I discovered that the "payment number" only works at certain convenience store chains, not including 7-11. For 7-11, I had to open a special page and print out an order form with a barcode that they can scan into the register. Joy.
I get that printed out and hike back to the store. This time it works, thank goodness.
At least I know my order is paid for, now. Oh, and one bright spot is that I was able to buy some packs of Korean-style Nori Tempura Chips, the taste sensation to which I am quickly becoming addicted, while I was there.