NOTE - the below are instructions for selling Products as upsells along with tickets. See here for selling a Standalone Product.
Go to the Box Office page
Select which Channel you'd like to sell tickets/products through
Select your Event and the performance you want to sell tickets for from the calendar
Add the ticket(s) you want to sell to your basket and click Continue to see Products
Add your Product(s) to the basket, either by clicking the + button or the 0 number to enter the amount you want adding to your basket
NOTE - if you can't see your Product or the price has not appeared, this might be because you haven't granted the correct Channel access or it has been unlinked to that performance.
6. Click Continue to process delivery, customer and payment details. NOTE - to sell Products as upsells, they must have the same Delivery Method activated as the Event. See here for more instructions.