Should I build this SAAS?

Background

My better half runs a facebook group helping other mothers with play / activity idea for their children, the activities are linked to development, it really good stuff.

Earlier this year she published a book on amazon, again related to play / activities for children. The book has done pretty well with her audience. After that she started putting together activity boxes which have a number of prepared activities within them. Again they sold pretty well.

This is where this idea was born, when selling the boxes she did so on her website, she doesn't have a large range so a full ecom set up would be overkill, especially as for a large part of the year she isn't actively selling anything.

So she made use of Paypal buttons to sell her products, this worked well enough.

The idea

Before I get into it, the idea is simple and it has been done before, numerous times in fact.

The idea is to allow her to add one click upsell to her sales flow. As I mentioned above, there are a lot of services that already off this, the issue I take with them is predominantly price. For her it doesnt make sense to sign up for one of the existing services we could find as they would all eat too far into her profits.

The flow we would be looking to achieve would be

Use clicks buy button
--> Fills card details, clicks buy
--> a page with a related product, asking if user would like to add to order
--> Click Yes --> redirected to thank you page.
--> Click No --> redirected to thank you page.

Email sent out for the order etc.

I believe having this simple function available to her will enable her to increase her average sale and increase her profits allowing her to grow her business more quickly.

At this point I think I can build her a solution for this, making use of stripe for payment processing, I will probably just do this whatever, it doesn't look like a crazy amount of work (But I'm terrible for underestimating things).

My ask

If I did build this would others be interested? If I did open this up beyond my partner I would look to keep the pricing low (well, low in my opinion, see poll).

Does something like this actually already exist at the price point(see poll)?