Thought of this idea for vouchers that can only show up after you've previously bought 2 specific vouchers during your run, and since this is generally harder than simply buying a voucher to be able to find its upgraded version, I thought it justified giving them some (generally) stronger effects.
Flash Sale (Overstock + Paint Brush)
+1 Booster Pack available in shop (to 3 Packs)
Marigold (Clearance Sale + Seed Money)
Earn interest for every $3 you have at end of round
Witchcraft (Hone + Magic Trick)
Playing cards in shop or standard packs can be Negative (+1 hand size while held)
Infinity Globe (Reroll Surplus + Crystal Ball)
Rerolling the shop has a 1 in 3 chance to restock a random Booster Pack (Must have room)
Moon Landing (Telescope + Director's Cut)
Cards played in your most played poker hand can't be debuffed by Boss Blinds
Leftovers (Grabber + Wasteful)
Permanently gain an additional +1 hand and +1 discard each round
Master Tycoon (Tarot Merchant + Planet Merchant)
Buying a Tarot or Planet Card from the shop immediately restocks a random joker in its place.
Dark Matter (Blank + Hieroglyph)
Wheel of Fortune has a small chance to add Negative to a joker. Ectoplasm no longer reduces hand size
I'm sure someone else could come up with their own fun combinations for vouchers, I just went with 8 pairs for the 16 base vouchers. If you have any ideas or feeback feel free to share!