One time sale on a custom product.

I'm not sure of what you want.
If you want to have your customers buy the "custom items", they have to be in HikaShop.

On our website, we have a specific product called "services" and which costs €5. When we do side projects for some clients, we give them a payment link for that product with the corresponding quantity. That payment link can be found in the "add to cart html button" button of the product edition page. When clicking on the link, it will add the product with the quantity parameter of the URL and display the checkout of your store so that the customer can directly pay.
Is that what you're talking about ?

Hi,

Yes, you can have an unpublished category and put the product in it. Then, give the URL of the add to cart html button of that product to your customer so that he can pay for it on your website.