Just in case you’ve not had an update, the 3rd party checkout system that was employed at Channel Advisor is being removed on the 18th May. You can see the full update and requirements here.
This is a forced change by eBay to move all buyers through a standardised checkout flow. Although, as far as I’m aware they have yet to provide an alternative that can deal with the complex shipping and tax requirements of some businesses and countries. A huge benefit outside of these two requirements for the 3rd party checkout system was the ability to offer credit card processing outside of PayPal, I suspect we’re unlikely to see anything threaten the eBay revenue cow ‘PayPal’.
Note: While there is a tool for very large retailers called municipal-level tax calculator its unlikely that “normal” sellers would be allowed access.
While personally a fan of the 3rd party checkout system myself for the reasons above and having the chance to actually alter a 3rd party checkout system to how I actually felt it should look like and feel at eSellerPro with fully customised and branded checkout experiences.
The only real advice I can offer is that is look upon this as a chance to simply processes. Complex is great, but sometimes simple is just as easy and much easier for customers to understand too. Ultimately this is a win for buyers as it gives a unified checkout flow and it does enable the eBay shopping basket that eBay have been trying to implement for 3 or so years.