When it says combined it just means you cant use two specials on the same card. Now on submissions you can have any size card or value and each can have a different special and all still can be on the same sub. The only thing that needs to be separate is the autographs but you can mail them in together.
Examples these were all on the same sub I had earlier this year-
$7.00 Wildcard $7.00 Thick Slab $15.00 Express / 10 Day $20.00 3 Day Special $12.00 Custom Circular Insert