Question about AT&T and adding a feature midcycle

Okay, taking a trip to Sweden next week. I'm adding the $25 20mb feature onto my plan 10 days into the billing cycle. I'll be back to the states before the billing cycle ends. I'm wondering how they'll charge me.

Will the $25 be added on to this next bill? Will I be charged from the point I activate it until 30 days (extending into next billing cycle?)

Their site says this:

 

Yeah, it's roaming data. I could go through the trouble of unlocking my phone then using a prepaid sim over there but I'd rather not go through the hassle.