Can they add items or just can't add items without being shown excessive postage?

Either send them an invoice with correct postage (requires them to purchase) or set up postage rules.

No idea if either works as I can't send invoices due to my business model and I don't use postage rules.

I get them to give me a list of what they want, work out a price including volume discounts (why I can't send invoices), set up a listing, they pay, I send.

Ps: I sincerely hope you are planning to add more items to your inventory. You're wasting a lot of money having a shop with 2 items listed.