Create item purchase dependency

Couple questions on purchasing items:

- I need to set a dependency on when people are purchasing items.    For example,  if I'm selling items A,B,C, and D people can only purchase A or B but not both.   Secondly, if they are purchasing item C or D then they need to have purchased item A or B.   I cannot figure out how to do this.

This not possible to achieve inside of payment field. 

You will have to create this using basic fields. You will have to split your items in two fields. For items A and B you can use Single Choice (Radio button) field. For items C and D you can use Single Choice field if you want that only one of those items can be selected. If you want that both items can be selected then you can use Multiple Choice (Check box) field. 

Then you will have to set conditional logic to show field with items C and D only when other field is filled. 

How to Show or Hide Fields Base on User's Answer

Here is my demo form: https://form.jotform.com/70663786323967 

Feel free to clone it. 

If you also want to perform a calculation please check this guide:

How to Perform Form Calculation Using a Widget

To pass a calculation to a payment field, please check this guide:

How to Pass a Calculation to a Payment Field

I have moved your other question to separate thread and we will provide you an answer there shortly. 

https://www.jotform.com/answers/1084776