I’m looking for a way to smart enable a heated mattress pad. Anyone have any previous experience???
You generally can’t smart enable an existing dumb pad, they will almost all have warnings not to operate remotely because of potential fire hazards.
There are a few Alexa enabled devices now available for sale in both US and UK that have added safety features that should generally be OK, although do your research before purchasing.
And you probably already know this, but if you are going to allow for automated control you only want to use an undersheet mattress pad that is fixed in place. Many fires start because a pad or blanket is folded over on itself when it gets turned on.
Once you have echo integration, you should be able to at least turn them on and off with an echo routine assuming you live in a country where those are available.
So the first thing we need to ask is what country you are in. 
Located in US
Dumb mattress pad with a switch and a dial, connected to a smart plug. That’s how I do it.
1) the whole mattress solution
OK. If you want to replace the whole mattress, Eight (originally called Luna) is popular, can set up heating zones, works with Alexa or Google Home or IFTTT (2nd generation or newer model). SmartThings Integration would probably be through IFTTT.
Their first generation models just weren’t very comfortable to sleep on, but the second generation have much better reviews.
https://site.eightsleep.com/pages/integrations
2) the (literal) vaporware solution
BedJet V3 is a cooling/heating system which sounds weird but seems to work. They’ve been promising IFTTT integration for awhile, but it’s not here yet. I would wait until they actually deliver.
3) the hacky IFTTT robot finger solution
One other option, but I emphasize again that for fire safety reasons this should only be used with a full mattress pad that stays fixed in place, not a heating pad or blanked that might be folded over, is to get one with a controller that has buttons that can be pressed by a “robot finger” like Switchbot. There are fewer of these than you might think because the Switchbot can’t do capacitive touchscreens and the button placement has to be far enough apart. Sugru will help fix the Switchbot at the right angle.
The only one I’ve seen that works pretty well and has good safety certifications is the Serra Sherpa mattress pad in this specific model, 860938 . The nice thing is on/off is a toggle, so you only need one Switchbot plus the Switchbot mini hub.
https://www.amazon.com/SwitchBot-app-controlled-device-mechanically-Midnight/dp/B07B4D9KVX/
https://www.amazon.com/SwitchBot-Hub-Mini-Smart-Remote/dp/B07TTH5TMW/
https://www.amazon.com/Sugru-Mouldable-Glue-Original-Formula/dp/B008MIRQUE
And here’s my recent review of the Switchbot:
So I know that’s a lot of different pieces for the project, and a fairly high cost ($175+$40+$40+$10=$265) and if you want to be able to adjust the heat rather than just on/off you’d be adding another 2 switchbots, but it should be both doable and relatively safe.
I don’t know whether to expect more devices in this category in the future, the fire safety issues make it a high risk item for manufacturers and a lot will just pass.
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Assuming the mattress heater has a button controller that is used to toggle on and off, a smart dry contact relay can be used to “press” the button. You get a high gauge (small diameter) two conductor wire and solder the wire to both sides of the button and via relay it connects these two as if the button is pushed.
My concern with this method is it may bypass safety features built into the heating pad. For example, if the heating pad has shut itself off because it’s overheating, the button remote is not supposed to work to turn it back on. But if you bypass it with the relay it might. Plus, of course, you will have violated the warranty on the remote. So I prefer the SwitchBot method, which simply literally simulates a finger press.
I was referring to controllers like this. Safety features like auto off etc still work because it’s pressing button same as human finger would.
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Is this what you are looking for?
Sunbeam Polyester Wi-Fi Connected Mattress Pad, Electric Blanket, 10 Heat Settings, Queen Size
It’s supposed to work with Google and Alexa.
You are responding to a post which is two years old, so I expect the original poster has either found what they want or moved on. 
Meanwhile, there is a more current thread which has included discussion of the sunbeam products. I myself have one and like it quite a bit. There’s no direct Integration with smartthings, but you can get partial integration by using Alexa routines as an intermediary.