Hey there! Totally hear your desire to get that device paid off, it's nice to do when you have the extra funds so that your monthly bill will be lower!
We have all the steps to make an EIP payment here: EIP Payments. Just double checking -- do you currently have any EIP Promotional Credits on your bill? We might want to look into which promotion they resulted from and make sure they'll move to the account level so that you don't lose them by paying your phone off early!
Manage your T-Mobile account