Allowable interest rate(s) for late invoices?

Generally, there is no limit unless related to real estate or personal, family or household loans. See the link for those limits. For other transactions:

Time payment contracts (for example: retail installment contracts and revolving accounts) are not generally regarded as loans. The usury laws normally do not apply to them. There are no limits on finance charges for the purchase of personal, family and household goods or services at this time. Banks take the position that the charge for third party credit cards (Visa, MasterCard, American Express, etc.) are not subject to these limitations.

In transactions for the purchase of goods or services which are not for personal, family or household purposes, there are normally no limits to finance charges except those set by the parties.

In the absence of an agreement, the rate of interest upon a loan or forbearance (an agreement not to collect until later for money already owed) of any money or goods or accounts (after demand) is 7% per year.

There really are no limits for several reasons.

1. California law does not regard the sale of goods to a buyer as a loan so usury laws do not apply.

2. A seller is permitted to charge a late fee in addition to any interest making the actual interest rate difficult to determine.

3. Most sellers offer discounts for prompt payment making the penalty for late payments even more difficult to determine.

The maximum is irrelevant if you have not provided for late charges or interest payment in the original sales contract or invoice. A customer need not pay these charges unless he agreed to them in advance. Your sales documents must specify the rates, conditions, terms and payments.

Should the debt not be paid at all however, you are then free to sue the customer for the unpaid bill and then seek reasonable interest, collection costs and out-of-pocket expenses in connection with collecting the debt.