How to hide products proposed as upsell from the whole list of products for sale on web store?

For that purpose you need to perform the following steps:

On admin panel of your web store on the page with a list of products hide the products which you want to use on "offer pop-up window" which appears when customer clicks on "Checkout" button on your web store.

To this effect you need to click the icon opposite the product required:

which will substitute for the icon:

2. On the app 8Upsell on the page of offer creation in the block Select product(s) to upsell choose those upsell-products which are indicated by the icon:

Such icon means that these products are hidden from customers on the list of products on web store, but will be available over "offer pop-up window".

The only condition is that hidden products used on the 8Upsell app mustn't contain different options of this product, i.е. the product must have only one variant of it. If your hidden product, which you want to include into offer, contains several options (for instance, clothes of different sizes or colors), so you can create beforehand several separate copies of such products without options, which will be absolutely similar, but will differ from it by name and photo probably. These limitations concern only hidden products, visible products used on 8Upsell app can contain options.