WP Fastest Cache stops estore products being added to cart

I recently set up WP eStore for my small online shop. All tested and worked fine, with a few orders coming in. Yesterday, however, someone alerted me that after adding a product to basket, the product didn’t show in the checkout page, which said ‘basket is empty’, and so couldn’t complete the purchase.
Once I cleared WP Fastest Cache, the customer was able to add the product and checkout with no further issues.
My question is: how do I prevent this from happening again without me knowing?
I am just an end-user, with zero technical knowledge, so I’d be truly grateful for step-by-step instructions.
Thank you!

The page I need help with: [log in to see the link]