PayPal will not process checkout payments that only cover shipping charges without an actual item total, so I think a better way of pricing cards would be 0.20 per card with 0.80 flat-rate shipping regardless of quantity. To do this, price your cards at 0.20, select the same Package Type (e.g. Envelope Regular) for all cards and enter their shipping Weight as 0.00 and Packing Capacity as 9999; then in Seller Admin > Cart Shipping Settings, enter the Regular Envelope Cost as 0.80.
Our "USPS Cheapest" rate-lookup method just obtains the First Class and Priority rates from USPS and applies whichever is cheaper (usually First class when available). That method is really just a legacy of the only USPS rate lookup method we originally supported years ago. You can create multiple rules using different calculation methods -- e.g., one using USPS Cheapest, another using USPS Media Mail, etc. -- and if a buyer's order qualifies for more then one rule, their cart will offer a choice of shipping rates/methods.
One caveat to bear in mind: our USPS rate lookups take into account the total weight of the order and the number of different parcels in the order (as determined by your products' selected Package Types and Capacities), and shipping a given weight in multiple parcels costs more than a single parcel. This means that an order for cards plus other items that ship in other Package Types would count as multiple parcels, which would calculate a rate higher than your actual shipping costs if you would ship the entire order as a single parcel together.
To avoid that problem, it would be simplest to regard the 0.80 card-shipping cost as a flat Handling fee for all orders. Then you would select the same Package Type for all products and set their Packing Capacity as 9999, and only enter an accurate per-unit Weight for non-card products.
