Halfway Compostela

It's not going to be easy, because they say in their web page that you must visit the Santuario de la Peregrina to obtain your certificate..
What you can try is to write to the tourism office of Sahagun ( oficinadeturismo@turismosahagun.com ) kindly explaining your case and adding that you live far away and cannot return to obtain it.
Add a copy of your credential showing the seals previous to Sahagun and ask them if as a special case they could send it to you at costs paid. If that's the case explain them that the Santuario de la Peregrina (the place where the certificate is obtained) was closed when you went there (it closes on Thursdays and the rest of the days opens from 9:30 to 14:00 and from 16.00 to 19:00)
That worked for me when I wanted to obtain the certificate of Padron and the office was closed too,
Maybe it will work for you.