If you host your site on "SiteGround" and using the "SG Optimzer" Plugin, you may encounter some issues with empty carts when visiting the offer page. This is due to aggressive page caching (which is a good thing!), but it can be easily resolved by requesting that WP Engine set up a special cache exception for your offer page.
You need to exclude offer pages by following these instructions:
Step 1: Note down the slug of your offer page from the general settings
Step 2: Open the settings of "SG Optimizer".
Step 3: Locate the option "Excluding URLs" on the left side.
Step 4: To exclude “offer” pages enter your noted offer page slug (from step 1) as shown in the screenshot: offer/*
Step 5: Click on Exclude button to add this slug as an exclusion.
Step 6: Here is the final output slug added of the offer page to exclude from the cache.
By default, the upsell pages slug is "offer". If you had changed the offer pages slug for any reason get the slug from WooFunnels > Settings > General > Permalinks.