He may be confused.
If he is posting on several different eBay's (we are on eBaydotCA, many sellers here also list on eBaydotCOM or eBaydotcodotUK, he cannot combine those sales. One site, one invoice, no combining.
What he can do is send multiple invoices, but only put the shipping cost on one.
For example, you buy three books from the same seller.
One on dotCA at $10CDN+ $5 CDN shipping
One on dotCOM at $10USD+$5USD shipping
One on dotcoUK at L 10 sterling.+L2.50 shipping.
The problem there is apparent.
However, the seller could put all the shipping on just one invoice and discount it -- for example charging only $7.50 USD on the US denominated invoice and saving by shipping the three books in the same box. The other two invoices would show Free Shipping, which would give him a boost in his DSRs, which are very important to sellers.
You could send this explanation to him if you like.
Oh the other common problem is easily solved. Tell him NOT to hit "Recalculate". It doesn't do what he thinks it will.