Agreed. Cheating in a relationship does not have the same definition as cheating in other contexts.
Infidelity (synonyms include cheating, straying, adultery, being unfaithful, two-timing, or having an affair) is a violation of a couple's emotional and/or sexual exclusivity that commonly results in feelings of anger, sexual jealousy, and rivalry.
Am I getting this right, you paid 20 USD for a plastic cover for plates in the microwave? I mean, yes, you should have one to avoid splatters, but I once upgraded from a 2 USD one to a 5 USD one due to better usability, I can't imagine paying 20 USD for it