Removing minor car dents from your vehicle is not difficult, and getting out a dent caused by an unguided shopping cart can be easily done. Shopping carts can sometimes accidentally bump the side of the car, and cause small dents. Although this is a collision-type dent, it is typically small and shallow, allowing you to remove it by yourself.
Using Dry Ice
This is a more traditional method, which can be done in an hour or so. Find some dry ice from your local garage, or a props store. Wearing gloves, or using a pair of barbecue tongs, touch the car dent with the dry ice. You can repeat this as often as you need to, until the dent is gone.
Use a Plunger
Take the sink plunger from your house, and use this to pull out the dent. Dip the plunger into some soapy water, and then press it to the dent. Pull the plunger, as if you were unblocking a sink, until the dent is removed.
Use a Hairdryer
If you can plug a hairdryer in to your garage, you can use this to get rid of your dent. You need to warm up the hairdryer, and then apply it to the car dent. While the metal is warm, spray compressed air onto it. The dent should then pop out.