Open Question: Does PayBySMS costs go into the cap/plan credit or is it additional cost?

Elaboration: So if I make a payment via PayBySMS, and I'm on a phone plan/cap ($30 cap - 3 Network - $150 credit) Does the cost go into my credit (as in $150 credit left, $140 after payment with the usual $30 bill) or as $150 before, $150 left after payment, and a $40 bill? I am on a 24 month-lock in, so I -think- that's a contract. In case you're wondering this is for Rune Scape, A fast answer would be very well appreciated. Thank you !