Great question aanugwom and thanks for posting.
I'm only seeing the option to store just one credit card online. However, there is an option to make a one time payment with a new card. There's even an option to enter in a certain dollar amount for folks that want to divvy up the bill.
tmo_mike_c's suggestion is a viable one. From my perspective, it would be easier to simply give your parents the cash (hand it to them, do a bank funds transfer, send them an old fashioned check, etc.).
I have 9 lines on my account, and I use a spreadsheet to keep track of charges and payments. It takes a few minutes, but with a price of $22/month/line, it doesn't bother me.