Shipping not always being added to total

Sometimes, but not always, the shipping amount displays but doesn’t get added to the total.

From trial and error I’ve narrowed this down to be caused by the Advanced Dynamic Pricing plugin. (Also disabling the plugin through the plugin manager causes a sitewide critical error, I had to change the name of the folder on the server to disable it without causing errors.)

You can see from the screenshot below, the subtotal is £35, the postage is showing correctly at £11.90 yet the final total is still £35. It is the same on the checkout page.

From what I can tell this only happens on products which are a fixed price and don’t have the dynamic pricing option.

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