How to let customers checkout for a custom built item?

I sell a few stock items, but mainly offer work on commission. I've got product pages for the stock items but need a way to regularly let customers pay me for a custom piece. Can I invoice them? Can I set up a product page that isn't visible to everyone and then adjust price details for each piece I sell, then send the customer a link? I know how to make my products, not much about how to program the web aspect of business. Thanks for any help.

SS isn't specifically set up to handle commissioned work. It doesn't have invoicing. I use PayPal for invoicing and it works well. If you've set up PayPal for your customers to make payments then you should be able to login to your PayPal account and get started with invoicing.

You could set up a second Store page in the Pages Not Linked area and add items there for commissioned items. Then give out the URL for specific product detail pages to each customer.

You may want to use my redirect to url after add to cart button has been clicked and SS indicates item has been added to cart code to redirect to the cart. It behaves like Express Checkout except you don't have to turn it on site-wide.

If you do set up a store per above you'll probably want to disable all the elements that would allow a customer to navigate to other pages in this hidden store. Also note that setting up a store in this way doesn't mean that those hidden pages won't get out on the internet in search engine results or that someone could find out the other pages in the store. There is a password setting that can be set per store page (the whole store, not each product detail pages within) but that probably isn't an extra step you want the customer to have to deal with.

Thanks Creedon. Shame SS doesn't really do this, as I suspect a huge number of small businesses would love to be able to give customers easy access to a custom option like this. I'll try the PayPal route first (wonder if Stripe could do something similar, so people could use credit cards?) first and if that doesn't seem smooth, I may have to consider switching to another service instead of Squarespace. Cheers!

Apparently Stripe does have invoicing.

PayPal accepts credit cards for invoices.