Not a common bug, but I've had it occur at least twice while using this service.

I've added the exact amount required for my labels into my One Balance.  The payment and the balance match exactly, but the dialogue says for me to add more money into One Balance.  The only option is to add another $10 to continue.

Edit:  the answer given is highly unsatisfactory.  This is a BUG that has existed for YEARS.