There are lots of pressure sensors that trigger at weights under 5 pounds, Or even under 1 pound, but they won’t be sold as a “Mat.” They are used in security and kitchen/cooking applications. Usually they’re just two pieces of fabric or plastic with the sensor in between. These are also called “force sensing resistors.” They’ll usually be listed as electronics or robotics parts
There is some discussion in the following thread:
And here are a couple of examples.
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https://www.amazon.com/Adafruit-Round-Force-Sensitive-Resistor-FSR/dp/B00XW2MIRQ/
Note that this is one of only three pieces that you will need. You need the sensor, you need to some kind of brain to interpret the reading from the sensor, and then you need something with the radio to send the event to SmartThings.
So doing it this way is one step more complicated than using a pressure Mat, which literally just closes a circuit when something is put on it.
The sensors I’m discussing in this post are intended for precision work, so they are providing a range of signal, which is why you need a brain device to interpret that signal.
LittleBits
If you want a simpler way, but much more expensive, to use the same type of device, littlebits does have a pressure sensor of this type. Littlebits are essentially Legos with electronics built-in, intended for kids, and could be put together in multiple configurations without soldering. You would need a power bit, the pressure sensor, and the “cloud bit” which uses Wi-Fi and has its own IFTTT channel. So it will be pretty simple to set up and integrate, but you would probably end up spending almost $100.
Or just use a WiFi scale
If you have room at your front door to put a delivery box, you could just put a wifi-enabled scale inside The box and use that. Some of these have IFTTT channels, which makes it really easy. Otherwise if you have an android phone, you can use Tasker and sharp tools to capture the scales app notifications.
I know several people who have done something like this, as long as signal can get out of the box it works fine and it’s very easy to set up. But again expensive.