Multi site vs single site

Looking for advice on the multi site setup vs single site. I have a site that offers repair services and products. I do not want to charge customers for repairs but only products. I want them to be able to add the repair to a cart and checkout as normal without payment on these repairs until work has been completed. i.e they schedule the repair thru the site and mail it to me. I repair it referencing their online order to charge them before sending back.

My thoughts were to use 2 carts so products and repairs cannot be added to the same cart to prevent checkout issues with payment or not.

My logic is to use WordPress as a multi-site configuration where the products for sale on one site and repairs on the other. I want the same users to access both sites without 2 different signup requirements. Basically the site should look like one site until products are added to the cart. Then 2 different checkout methods. i.e when a search is performed it searches both sites.

Is my method sound or is there a better way?

Thanks in advance for any advice.