I have checked one of the parcel services - europaczka.pl - they claim to be cheap - but shipping your box according to them will cost you circa 300 PLN (they have an online calculator)
See threads here on them - they are not good, though less likely to steal something going out rather than coming in. About the Spanish postal service, anybody's guess. Also, the charges for sending 25kg to Spain will be high.
there used be an alternative before - I used it to ship a parcel from London to Gdańsk back in 2004 - Polish coach companies used to allow shipment of parcels that then had to be picked up by someone at the arrival of the coach
I checked that there are direct coaches from Kraków to Barcelona - maybe you should talk to the drivers - the Sindbad coach departs two times a week (saturdays and wednesdays) at 11 o'clock from here: KRAKÓW, Dworzec MDA, ul. Bosacka 18
or you can call Sindbad infoline 801 22 33 44 - they should talk English on the infoline - and ask if they accept parcel shipments
I had money and parcels send by the Poczta Polska and I never had any problems. On their site they say: 149 zl for 20 kg and 89 for up to 6. So you'd have to send 2. That is with confirmation that the parcel was received