Please correct me if I am wrong. If it states pick up only, why is it the Sellers problem to find out rates to courier or pack & send or post in any way? Should 'nt  that be the  Buyers responsibility? Why would  you not  just nicely remind the Buyer of your TOS and say you are however, very happy to work in with their courier or freight company of choice?    Forgive me, I am only a few months into Selling so I might be way off base.:8}

Way off base? NO, you are spot on.

While some might come here cussing and cursing about buyers most of us do reply politely and your suggestion for a reply is most definitely polite but firm, just what is needed.