What would happen if I fail to pay?
Chances are that they will process you for court, obtain a judgment against you, and try to collect by means such as garnishment of wages if you do have employment in the USA. Of course it will hit your credit at some point (if you have a social security number) and your credit score will take a deep dive and graveyard spiral into the dumpster.
Would it affect me later on if I apply for a ESTA visa for a visit?
NO. Grounds of Inadmissibility per INA: ACT 212 - GENERAL CLASSES OF ALIENS INELIGIBLE TO RECEIVE VISAS AND INELIGIBLE FOR ADMISSION; WAIVERS OF INADMISSIBILLITY Sec. 212. [8 U.S.C. 1182] is what is used to determine your eligibility or otherwise for a visa/ESTA (immigration benefit).
In this case we will be looking under inadmissibility due to criminal reasons. Mainly here we would be looking if you committed fraud which would in many cases be a Crime Involving Moral Turpitude (CIMT) that make you inadmissible if it doesn't qualify for the Petty Offense Exception under 212(a)(2)(A). To stretch this civil case into the realms of fraud is highly unlikely by any prosecutor considering the amount involved < $10k.