Unautherized charges attracting interest

I cannot understand why I constantly have to fight about this matter.

I bought a phone on a 6-month revolving payment plan (interest-free). But every month on the account statements I get a charge of random amounts of interest. My statement has card purchases and same-day credits on those same amounts for no particular reason—obviously all these unauthorized charges and credits TFG is doing without my consent are attracting interest. Even a new charge on a magazine subscription I never heard of.

Refund all that interest today and close the account—I'm done with TFG store accounts.