Did you eventually download it to the iPad? You cannot trensfer it from the iPhone, it has to be redownloaded from Apple directly to the device that will be using it.
Do you see any error messages that you can share? Are you sure it is com[patible with an iPad; specifically the model you have?
If you're not getting any satisfaction from the vendor and wish to ask for a refund, here's how to request an app or media refund from Apple (From: Request a refund for apps or content that you bought from Apple - Apple Support - see the article for more detail.)
1. Sign in to https://reportaproblem.apple.com
2. Tap or click "I'd like to," then choose "Request a refund."
3. Choose the reason why you want a refund, then choose Next.
4. Choose the app, subscription, or other item, then choose Submit. If you were charged for a subscription that you no longer want, you can also cancel the subscription using the directions in this article --> If you want to cancel a subscription from Apple - Apple Support