You don't need to use a courier.
I send POAs and many other legal and tax documents both ways using recorded post. In Spanish the service is certificado and if urgent then urgente + certificado. For a POA it will probably cost EUR10.
The Foreign & Commonwealth Office now send legalised documents via Fedex these days but up until recently they were also just using recorded delivery.
I even send withholding tax certificates through the post and if they go missing then you might have problems but they don't.
As for better priced couriers you could try MRW.
But you should consider having POA revoked especially if executed in Spain.