I recently bought an e-book on Gumroad and I had a discount code. However, during checkout, there was no way to enter the code. Apparently, that field can be disabled.

Gumroad checkout page of a product without a discount code field, and a checkout page of a product with a discount code field

But I found a way to apply the code nonetheless: Just add the discount code to the Gumroad URL:

http://gum.co/{ProductID}

becomes:

http://gum.co/{ProductID}/{DiscountCode}

You can then immediately see the updated price:

Browser window with a Gumroad product page open showing a price tag of $18, beside a browser window with a Gumroad product page open with a discount code appended to the URL. The price tag shows a reduced price of $12

That’s it! For more details, you can see the official documentation on Gumroad.