Where is the redemption code, email just has a redeem link but cost is £50.
This fooled me as well. The process is this:
Follow the link, log in to your Tesla account and select either the mud flap kit or the paint protection film
Go to the start of the checkout process and put in your details
Click through this and there's an option to add a voucher code. Cut and paste the code from the email, editing it to remove the asterisk at the end.
The price should drop to zero after the voucher is applied.
FWIW I ordered the paint protection film, as I already have mudflaps fitted.