Decrptive billing practices

What is the name of your state (only U.S. law)? AZ, however the billing originates from MO

My life insurance company has been billing me for semi-annual payments of various amounts.
I have discovered that when they billed me $73.50 for four semi-annual payments, they credited
the payments as five month payments. When they billed $80.10 for a semi-annual payment, they
credited it as five monthly payments plus a $6.60 credit. Both methods result in $14.70 a month
payments which completely negates the advantage of a semi-annual payment. That amount is what
they bill as a regular monthly payment

What is the law on deceptive billing practices? The company offices are located in Plano, TX, but
the payments go to St. Louis, MO. The company is a subsidiary of AEGON.

Thank you.

I guess I'm misunderstanding...

You need to pay a total of XXX per year. You pay a total of XXX per year. What is the concern?

Call them and explain THEIR error. They should correct them as it is obvious what your intent was.

It is sometimes the case that paying for more months at a time will save you money but that is certainly not always the case. What is the basis of your belief that you will be billed less for paying for a longer period at a time?