This post is from a few days ago. Sorry you didn't get a response sooner @gladsong777 - do you still need help?
There are many ways to do this from setting up combined shipping rules to having the buyer request an invoice through the shopping cart to having the buyer make a purchase then refunding excess shipping after the fact.
Setting up combined shipping rules on your listings is the easiest method as shipping can be automatically combined based on weight or with flat ship rules through the shopping cart.
Read more on this eBay help page:
https://www.ebay.com/help/selling/shipping-items/shipping-rates/offering-combined-shipping?id=4169
