Regardless of how you want to calculate the interest, I have found this which does appear to allow the seller to demand interest payment presuming no interest was included in the terms of the contract
Texas finance code 302.002
so, while it appears the interest rate charged far exceeds the allowable rate, to determine that definitively one would need the terms of the original contract.
Again, Iím still not sure this even applies. There is a difference between a loan and an installment payment plan, especially since title was not transferred until the payment in full was made. I donít know if it takes it outside the perview of the finance laws. I suspect it may.