Yes. If the Review & Confirm step shows an insufficient balance, click one of the Add $50 / Add $75 / Add $100 buttons. A prompt tells you the exact charge including the 3.5% fee ("You will be charged $51.75 (includes 3.5% processing fee) for $50.00 in SMS credits. Continue?"). After a successful charge the page reloads and the confirmation is re-evaluated so you can confirm the send.