If possible I dodge the dealers who have unrealistic p&p. If you need to deal with someone who sells cheap and balances the cost with high p&p, just decide whether the product is worth the total. To me it does imply a certain attitude and that appears to be the case by the reply you were given in this case.
I wouldn't loose sleep over it, though.
However, I had this situation a while ago and each product dropped through the letter box in its own separate package in the same delivery so it may just be easy for the trader not to bother to link items.