well a post office shouldn't be that expensive! i once used ecoparcel.eu, its rather cheap and they took my package from home so i didnt need to go to the post office;)
Related:
What are the best/reliable/cheap Parcel Deliveries to use in Poland to UKI'm moving back to London from Lublin and wanted to know what is the best way to send my suitcases back to London. Door to door services? I have 3-4 suit cases of 30kg plus in each.
APJ Transport. They charge £1.20 per K.
I use them from Manchester to Lublin alot of the time.
From London its should be cheaper, 90p to £1 per kg.
Eg catrinatrans.co.uk/english