United Mileage Plus has its own promotion for additional miles (now that AAdvantage went ahead with their promotion):
Earn up to 2,000 bonus miles per rental with Avis.
Choose Avis for your next trip and earn up to 2,000 bonus award miles*. You'll also earn 75 miles per day along with saving up to 35% on every car you rent.
Plus, enjoy an additional 150 bonus miles per rental when you book on united.com
Hurry, begin your rental by August 31, 2014 to take advantage of this special offer.
Discount code: AWD K019385
Coupon code: MUAA009
The way this is written may suggest that you can use any AWD (Avis Discount Code). This has been the case in the past at least.
Make sure you add this to your reservation – Coupon Code MUAA009 .
Also, remember Avis rentals only earn miles at airport locations!
You can book your Avis car here.