As you know, you are unable to refund your customers as the purchase was made through the Asset Store. However customers will contact you when they are requesting a refund on your asset.
You should help your customers in any way possible to help resolve their issue to avoid a refund. If a refund is required then you must request it on the customers behalf: this is done by the following process:
Ask the user provide you with the invoice they received on purchase of the asset.
Login to your publisher administration page.
Verify the invoice using the Verify Invoice tab.
Once verified, e-mail assetstore@unity3d.com from any e-mail address attached to your publisher account. You must provide either the invoice as an attachment, or the invoice number.
Here is a template request:
Hi Asset Store Team,
Please refund the following asset(s)
Publisher Name:
- Asset name(s)
Regards