I don't recommend this as a deliberate strategy, but last year I was able to use coupons with E-fares. Apparently the main credit card I had registered with the site had just expired, so United.com replied to the booking with a message to go to a CTO to complete the res. When I went in person, the agent was able to easily pull up the res and when I offered up travel credits, she took them with a smile.

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UA 1K, AA Plat, CO Gold. Every mile flown comes out of my own pocket.