Formula - Net amount from multiple options

Formula - Net amount from multiple options

I have tried, and (probably stupidly) failed to create a formula that fulfils the following function. It is really a postage type formula.

The following fields must be filled in by the sender:

Letter type:
*Postcard
*Letter
*Package

Letter size:
*small
*large
*packet

Location:
*UK
*Abroad - Europe
*Abroad - World

I would like a "net value/total cost" type field to update depending on the combinations of options chosen. Apologies if similar questions have been asked before, but I would appreciate any help,

Many thanks!