I need the form to add up a number of items, not just one with a quantity

I need the form to add up a number of items, not just one with a quantity.

t shirt 10

shoes 20

bag 50

If I want to buy 1 of each... then calculate $80 with GST

You can use an Inventory widget for each of your products as shown below:

You can choose to show how many items are remaining on each product or choose to hide it as shown above.

After that add a Form Calculation widget and add these Inventory fields into it:

Related Guide: https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

Here is a demo form: https://form.jotform.com/200778060043044 

You can clone it into your account if required: https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL 

Do review and let us know if this works for you.