When a customer uses the Promotional Kit with the Promote Web App enabled, it will not display a short URL link. Actually, in the promo Kit, you will see a long URL displayed when choosing Promote Web App and selecting the Show Shortlink URL options.
This article provides alternative methods to create a short URL for the Web App link.
1. The workaround would be to use one of the many alternatives available on the web to generate a short URL. The following two sites appear to work really well and are really easy to use:
2. As you are in the Grow -> Promote -> Promotional Kit 'Edit' mode, select from the right pane the Custom URL radio button.
3. Paste the short URL you got in step #1, in the field underneath
4. Click on Save on the upper hand right corner.
You will be able to access the promotional kit through a short URL.