All items in the shop should have downsides as well as upsides and should not be straight upgrades. They should all cost the same ammount of money, and should all be able to be obtained through non shop means, such as by flying through certain randomized rings, which can appear and move position each round. This means that the gameplay has good balance, and a player with no items can still win over a player with items. If you play your cards right, this also means that nothing in the shop is a straight "upgrade" or "downgrade" meaning each weapon or item is completely viable no matter of situation.
Another idea is to just make loadouts selectable and all items free. This would also be fine and would offer all the upsides i have mensioned while you not needing to waste your time making a shop or a randomized item ring system.