I purchased the 50-135 from Adorama some time back, the extended warranty they were selling was through Mack. It was about 35 dollars for 7 years. I have not had to use it. Whew! But you never know. I would never have bought this lens without an extended warranty. They still sell it for the same price periodically. However the cost of the lens varies widely. If they are priced higher, I am sure they will match it.
Mack Camera Repair Mfr# 1017 SKU: CZW7L29
Mack 7 Year Warranty for Lenses Valued $600.00 to $1000.00.
$34.95
Some time back there was a discussion on this forum on Mack warranties.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/131551-mack-ex...arranties.html
I am not sure there will be much difference between Mack and Sagemax. In fact these companies simply contract the work out to third party repair shops. I think someone on this forum said that when they send it to Mack, it was actually send to the official pentax repair center for repair! But in a particular case you never know who will be doing the actual repair.
Also someone in this forum actually works for Mack.