Hello Cuda.
Are you employed at Delta?
Stockholm is a very popular destination in jun-jul-aug. You must expect a lot of sold out flights.
I work for SAS-SK and when I travel nonrev with other airlines I normally have to use a paper ticket that I get from our company ID office.
We normally pay a little extra using NON REV on other airlines but is is normally not a big difference. Maybe 50USD extra for a ticket OSL-EWR-OSL.