Today, when trying to pay for a flight with an eCredit, Delta.com takes me to the screen where I can see all three of mine listed. The line at the top tells me to select the one I want to use, and then click continue.

However, there is no button to select a voucher, and no continue button shown (perhaps it pops up after you select your voucher, but I'll never know). I had to call in and had an agent book my ticket. Luckily, she knew how to waive the $25 fee that the system automatically charges to PM/DMs using vouchers if you don't get an agent that waives it correctly.