Contractually you need to quote your fees and state the time within which payment is due.Make the quotation in a written instrument. It will probably extend the statutute of limitations depending upon the laws of your state. It will also help you with the rules of evidence if you need to file suit.  If they send you work after having received your quotation they have given you an acceptance of your quotation or alternately ratified it. There are a number of contract law theories under which you could recover damages.

The problem with Valu America is that it does need some exploration as to what services they provide for which you are being charged, the correct amount of their charge and whether your fee can be adjusted to compensate for it. It is insane to agree to their terms until you know exactly what the terms are.

Remember until you know the relationship between ValuAmerica and the client (agent, subvendor, what?) you are dealing with two separate entities. You need to clarify the relationship between the two.