I hate asking for money. I'd email them and say "I'm so sorry! I forgot to check with you about how/when you wanted to pay for your vacation week. Did you want to drop it off, or just do an email money transfer? I'll be around this evening from x:00 to xx:00 if you'd like to drop it off. Thanks!!"
For most of my parents, it really has just been a matter of them forgetting. Or, if they really thought they didn't have to pay, they've all been polite enough to fake that they forgot about it
. It usually only has to happen once, and they've always rushed over to drop it off as soon as possible. Usually
.