I can only tell what I’ve learned in my forays into billing and small clinic. Generally Providers have agreements with commercial Insurance which the copay can be fixed to a certain amount depending on policy patient carries. This is also true with say, U.S medicare Advantage private coinsurance. While copay in OpenEMR does not have to be a given amount at any visit, it can be set to a default copay depending on their policy/payer. At checkout/payment you may enter the amount collected for a copay and use payment option copay. This will setup as a payment for posting. The percentages will need to be up to the person taking payment. I don’t know that OpenEMR allows for maximum’s here but jeez, I’ve been fooled before:)
For billing insurance, depending on insurance company, there will generally be a set amount payer will pay against various billed procedures regardless of the amount you bill the payer, so in that regard, a max amount.
Maybe this helps some.