When did you buy the Series 1 w/lifetime? If you bought it before January 20, 2000, you can transfer the lifetime service to a new TiVo for free.
Other than that, the ebay discount codes are probably where you will find the best deal. And if you'd rather not deal with the ebay codes, the "PLSR" code you found will also get you $100 off lifetime.
You can also check out the "Roamio Deals" thread:
Some older TiVo customers (over 10 years) have recently reported in that thread about getting a Roamio Plus w/lifetime for $500 when they call and ask for it.