Call PV and ask them
This is from Midsouths website:
"Frequently Asked Questions
Information about orders Gunpowder & primers
All orders containing gunpowder, primers, or percussion caps will charged a $25.00 hazardous materials charge per shipping box with these items. If shipped by air, then primers, percussion caps, and loaded ammunition will be charged $35.00 per box. Gunpowder cannot be shipped by air. Powder and primers can be packed together on ground shipments for one hazardous charge provided the total weight of the order (including packaging) remains below 70 lbs." That is their policy. Others may not let you combine. You really need to decide what you want, and then call them. There is no obligation to buy on the phone.
Your other question about order quantities to be worthwhile: You need to price the same items locally at sporting goods store and gun clubs. I have found that I need to order several thousand primers and several pounds of powder, at a time, to break-even. So, I watch for "on-sale" primer prices locally. or pay too much for one pound of Varget at the gun club.