How to combine two refill cards?

[bosthy]

I've just bought a new prepaid phone and I would like to activate it using the $45 unlimited talk & text plan.

I already have a $15 refill card, so I'd like to use it first and then pay the remaining $30 with another refill card or my credit card directly.

The activation page doesn't let me do that. It says: "Please enter Refill Card with balance amount equal to or higher than the total due amount."