Your shop
Sell water, grips and shirts in the same place you sell a membership.
A product is a product
Something you sell sits in the same catalogue as your memberships and class packs.
The same checkout
Same till, same cards, same option to split it over months.
The same invoice
A shirt gets a tax invoice exactly like a membership does.
At the desk too
Charge an item to the card a member already has saved.
Stock is not counted yet. You can sell things; you cannot yet see how many are left.
Everything a studio runs
If one of the three costs you money, tell us how much.
The first conversation is questions, not a deck.