Both buyers and sellers may have specific instructions on how to ship a particular item. As such, there are no hard and fast rules but there are some good practises to adhere by. Unless otherwise stated, it is usual for the buyer to be responsible for additional shipping costs.
Firstly, discuss any special requirements and be aware of common practise for the collectible in question. Some items may require insurance and others may mandate lots of damage protection (for example carded action figures). All of these factors influence the final shipping costs.
Lastly, you may need to consider multiple shipping costs and state these on your listing dependent of where the destination is. Shipping within country will not be the same price as shipping internationally. International costs can quickly add up when customs and other duties are piled on top.
Once all the finer details are worked out, it is a matter of contacting your couriers to discover pricing. This is usually a function of how heavy the item is, the dimensions needed to package it safely, and if any add-ons like insurance are required. It is usually a very good idea to state on your listing which courier you intend to use. For example, in the U.K. that might be Royal Mail, in the U.S.A it could be UPS.
Once you have been through this a few times, it does become second nature and you quite often figure out where you can save costs without compromising on your service to the buyer.