“3 gallons of each product”
(POR and House paint)?

I'd get 3 gallons of each product. If you can buy them locally, you can return unopened can/cans.

You should be able to get the house paint "pre-mixed" (off-the shelf).
An unopened can should be accepted as a return?

Can you find the POR locally - if yes, an unopened can shold be accepted on a return.

I have only done this on a 19' flat bed truck.