Regards,
Alex
This often happens, I've had similar experience some time ago. It's also possible that the error message is indicating that you don't have enough funds to cover both the transaction fees and the associated mining fee. The few times I paid with ETA, I figured out that this often occurs because the estimated amount isn't always enough to cover for both the transaction fee as well as the mining fee. In this case you can try increasing the amount you're paying or you can also try to adjust the fees and see if it works.
If this persists you could try an alternative payment method, I'll highly recommend STATIC, unlike ETA static payment method will guarantee the exact amount you're paying and you can as well set an actual fee, I believe you wouldn't encounter such problem with STATIC