For what its worth I recently closed my Serve card after the following experience. I loaded about $600 using my CC and used my card for a few small purchases. I then used a free ATM to pull $100. I then used the bill pay function to pay a CC bill, different than the one that I used to load the account with (I did about $300 on the CC bill payment). The next time I loaded the account I received a notification that I could not use that card (CC) to load the account and I needed to call Customer Service.
I was able to load using a US Bank Visa Bux card which is a debt and I was able to make purchases. I never called Customer Service. I paid the remaining money in the account to one last bill pay and closed the account.
Just wanted to add a data point of using a CC to load Serve.