A lot of the larger machines can be very expensive!! I got a smaller handheld one that still works just as well as I need it to and multiple settings. I have pretty severe Endo, though I'm also missing a disc in my lower back, I have 2 herniated discs and another deteriorating, and I use the TENS 7000 all the time for back pain as well. This helps me just as much as the professional unit in the physical therapy office. Not quite as many settings as that one, but it gets the job done! You just need to change out batteries when they die and buy new pads if the stickyness wears down (though I'm only noticing I need new pads after having this unit for about a year with moderate use).

They are selling the newer version of the one I use on Amazon for about $27 right now, and a 20-pack of replacement pads for about $13. I provided links:

https://www.amazon.com/TENS-7000-Digital-Unit-a...(Phone number can only be seen by the question and answer creators)&sr=8-3&pi=AC_SX236_SY340_FMwebp_QL65&keywords=tens+7000+unit&dpPl=1&dpID=41a85-5QkSL&ref=plSrch

https://www.amazon.com/gp/aw/d/B079G8PDR4/ref=m...(Phone number can only be seen by the question and answer creators)&sr=8-3&keywords=tens+7000+unit+replacement+pads&dpPl=1&dpID=51LklZvFHhL&ref=plSrch

Idk but I would be interested, I'm looking into getting one.