membership at the end of the month?

neminem thought this was worth mentioning said
2

I just want to confirm, since I’m thinking about buying a pair or two of those headphones, and shipping on one item is the same as membership for a month, so that seems like a no-brainer in case I happen to see anything else I want during the month: am I reading it correctly that if you paid for membership, the first month you’d only have until the end of the same month, that if you joined on the 30th, you shouldn’t even bother? That doesn’t seem right…