Note that some, but not all, international sellers may be prohibited from combining shipping according to eBay rules or for other reasons beyond the seller's control. If a seller is prohibited from combining shipping for international orders, there is very little a seller can do to fix that.
One of the reasons sellers in some locations are prohibited from combining shipping is due to the very complicated way import taxes and customs duties is calculated for commercial products being shipped to buyers in other countries. Complying with the requirements of both the seller's own country and the buyer's country too, and complying with wildly varying import taxes, customs duties and other fees from country to country can complicate international shipping to the breaking point for eBay or the sellers themselves.
If the seller is prohibited from offering you combined shipping by eBay or unwilling or unable to combine shipping for other reasons, the seller can respectfully decline your request and explain why without you losing any money.
Not saying that's what happened to you specifically, but note that it is a common practice among sellers to ban buyers who leave anything other than positive feedback. This is especially true for buyers who punish sellers for complying with eBay rules or for other matters beyond the seller's control.
In other words, for example, if eBay rules prevent a seller from combining shipping for an international order, and a buyer cites lack of combined shipping as the reason for neutral or negative feedback, the seller is very likely to ban the buyer.
Likewise, if a seller does in fact combine shipping after the buyer has requested the seller do so, then the buyer still provides neutral or negative feedback, the seller is likely to ban the buyer.
Language barriers often complicate buyer-seller communication. Multiple requests, patience and persistence may be required to ensure the buyer gets what he or she wants at a competitive price while at the same time the seller is paid a fair price for the product and service they can provide.
Note, too, that sellers can "un-ban" a buyer as easily as they can ban one. If you want to continue to buy from a seller who banned you after you left less than positive feedback, contact the seller and offer to change your feedback rating to a positive rating in return for the seller lifting the ban.
Many sellers will lift a ban for a buyer willing to improve their feedback rating.
Sellers want to sell and buyers want to buy, but some negotiating and compromise may be needed to smooth things over after a buyer-seller experience went bad.
Hope this helps.