Buyers cannot change the shipping charges that you enter. If that were the case it would be happening to every seller. I've been selling on this platform for 20 years and it's never happened to me but you are a new seller so I'm presuming it was actually an error on your part. When you create your listing you have to select the method to ship, first class, parcel select, priority mail, etc. You also enter the weight and size of the container you will use. That's how the shipping is decided. It's not possible for the buyer to simply enter their own weight/size or price for shipping. That wouldn't make any sense.
I suggest you go back to look at your listings and figure out what you were missing so you can learn from that and set it up correctly. Your buyers have to pay whatever you select for shipping. You as the seller will make that choice. Best of luck to you....
