A debt collector, who is also a law office, has been harassing me for a year now for payment of suspicious services with which I do not agree. I paid in full in December 2017 ($506.66) by money order and now received a further invoice of $1056.72.

Tell them you don't believe you owe this debt ask them for their business license number and address and any and all information they have on this questionable debt and see what they send you and then use that information to file complaint with the Better Business Bureau and do not give any of your proof just file a complaint about them not leaving you alone

It is illegal to harass you over a debt ask them for a mailing and physical address and send letters asking them to stop harassing you. And if they threaten jail or fines ask for the physical legal documents.