Have a look at the Replace Field Contents command/script step.
Note that the problem with your method is that when one user selects a product, it is selected for all other users as well. And there is no history of the quotes produced.
I agree with Michael. If this is a multi-user solution, checkboxes won't work.
Use a global variable to store the IDs of the selected items instead. Replace your checkboxes with a button that looks like a check box. That button toggles the values of the global variable. You can conditionally format the button to indicate whether the record's ID is in the list or not.
The database is not running off a server and it doesn't matter if other users selects the same product.. This is only to generate a proposal which is then sent as a pdf to a selected email address. (the office staff then process the orders and check stocks) This is just to speed up the process capturing the order in the field and giving a better visual display of the product and details. Once the the pdf has been generated and emailled the checkboxes need to be cleared ready for the next client.