I have a great deal of experience launching new advertising platforms, and I can tell you it is not easy. Key levers for driving advertiser adoption are Reach, Targeting, and Performance (ROI).
When launching a new platform, you face a chicken and egg problem. Advertisers (Demand) will be most interested to buy with you if you have a large addressable base of users who can view the ads (Reach). Suppliers (typically content publishers, but in your case shippers printing labels) will be willing to go through the integration efforts if you have a large demand set to quickly bring them revenue.
Once you have supply in place, you will need to demonstrate ROI (Performance). That means performing, and publishing case studies which highlight success for your advertising clients. Any campaigns that you run will need clear measurement to evaluate the ROI whether through call-to-action results or follow up consumer surveys.
Ultimately, the price you can charge to advertisers will be driven by their ROI (or perceived ROI) and you’ll need to evaluate whether this sufficient to cover your cost of inventory and overhead.
The ability to finely target desirable consumer audiences is a strong lever to get advertisers to buy. I would be sure to work closely with your shippers, and likely a third party data source to identify the consumer type receiving the packages. The good news is that the direct mail industry is rich in consumer data.
The ad unit (shipping label) that you deliver may not be compelling to buyers. You may need to work with your shippers to enable including an insert in the packaging itself. Fragrance Brands may find this appealing to deliver scent-strips, for instance, as consumer sampling is an important marketing goal for them.
As a final thought, you can try to leverage the novelty of what you are doing. There is a great deal of excitement (and hype) right now around native advertising. There are many definitions of native advertising out there, you can certainly create your own. Perhaps call it “Native On-Box” advertising, and do some surveys to gather metrics on the high recall rate, or “open rate”, or favorable brand association with the excitement of receiving a package, etc.
Bottom line is, you’ll have to build interest from the demand and supply side simultaneously, and possibly subsidize your first campaigns to entice both sides to participate. You’ll need to gather key metrics to point to ROI and turn those into case studies that resonate with advertisers.You’ll need to put in place the targeting to make the ad spend efficient. If you have inventory, you can likely get pay-for-performance campaigns right away. It may be difficult to make them pay, but you can at least, easily evaluate their performance. Pay per lead campaigns are readily available from a large number of brokers.