Do I need a separate co-packer to apply shrink sleeves?

Often yes. Printing the sleeve and applying it are two operations — application needs a sleeve applicator and a steam or hot-air shrink tunnel. Some printers offer turnkey print-and-apply, while others ship flat sleeves to your filler. AJ Cosmo Labs lists both shrink sleeve manufacturers and contract labelers that handle application, so you can coordinate the full workflow in one app.