Not sure how many of you care about this feature but I was testing shipstation for the FBA module and found a way better solution.
Amazon offers the ability to add prime on your website (not webstore) and gives a discount for doing so.
I am currently working on the integration but all you need is checkout by amazon and you activate the prime feature. You then add a widgit on each product page that is FBA item, when a order comes in using checkout by amazon and the customer is using the prime option the order is sent straight to amazon.
The best part about this feature is you save $3 on each multi channel order when a customer uses the checkout by amazon on your website with the prime account feature on the product detail page.
With 11 million users on prime I find it very important to offer it on the products that are eligible on my site. The number will continue to grow and giving this option on your site is a good tool to push buyers to order with their prime details, now the customer feels they can use the prime account outside amazon.
You can still use shipstation for people that dont use the checkout by amazon feature and use a prime account but wanted to share with you the massive $3 savings, They give you the same rate as if it was sold on amazon for a multi channel item.
You can read more about it here.
Participating sellers can enable Amazon Prime on Your Site for free. While all standard fees for Amazon Webstore, Checkout by Amazon and Fulfillment by Amazon still apply, sellers adding Amazon Prime on Your Site will be able to take advantage of a $3 discount on FBA Multi-Channel Fulfillment rates for orders containing at least one item fulfilled through Amazon Prime on Your Site
https://amazonpayments.s3.amazonaws.com/documents/Checkout_by_Amazon_Prime_US.pdf