Fix WooCommerce Checkout Failures

I run https://ldvpartsdirect.com/, a WordPress-WooCommerce store for LDV car parts. Stock control is handled with the ATUM Inventory Management plug-in and payments go through the TYL banking gateway.
... (Budget: £250 - £750 GBP, Jobs: API Integration, Debugging, MySQL, PHP, Web Development, Website Optimization, WooCommerce, WordPress)