It works when the partner is set up for it, and falls apart when you hand off to a shop that treats you like a retail customer. The things to check: do they produce in-house or subcontract further, will they match your material specs, and will everything ship unbranded so your client never sees them. Also agree turnaround times in writing up front, because overflow work is by definition urgent and verbal promises don't survive a busy month. Promo Box LLC runs an explicit white-label program for wrap shops and agencies, see
promoboxllc.com - Everett based, in-house production since 2008. Start with one job before committing your busy season to anyone.