I have been doing this for years, just not to the extent you are doing it. I actually offload gift cards to my insurance and their systems just automatically send me a check every month. I only do a few grand a month. I actually think what I do is more annoying to them than one 25k charge per month. I just get more miles by buying gift cards. Your miles are free. I think you are ok. Ayway there are many insurance compnaies, and many that allow overpayment. One caveat - Once with Travelers, they took my overpayment and applied it to someones elses account that had the esame street address as mine who was past due. i got the money back and they askeed why I kept prepaying. I simple told them I wanted to budget my annual fee without payiong their installment plans. I did get my $ back after a few months. I don't think my argument would work with 25k monthly overpayments.