Once bigger than large letter then the zone rates apply.
This can be a pain as same applies to Worldwide.
Without ebay making changes which allows more postage rates to be shown, I can only suggest quoting the highest zone postage for all items, if then buyer overpays, you can always refund excess, and as ebay also refunds the fee on the amount refunded, you do not end up out of pocket. A brief message to buyer explaining what you have done should avoid problems.
