SKULabs currently has highly customizable packing slip customizations but the providers SKULabs works with do not currently offer shipping label customization because this is against carrier regulations.
SKULabs
Within SKULabs, access your brand settings under each store. All branding settings are located within each store so you can operate each of your brands independently without overlapping any resources between brands. Customize logos, packing slips, and emails per-store.
EasyPost
As a licensed PC Postage provider for the USPS, Easypost is actually required by the USPS to have the EasyPost logo on all the labels. If you'd like to introduce your branding to the shipping stage of your package, Easypost offers a tracking page that you can insert your company logo to. Just go to your dashboard, click on "Brand", and upload your logo. For more information, contact Easypost at [email protected].
Shippo
Shippo also offers branding options.