Samsung normally will refer you to ubreakifix.com - - I believe they can fix just about anything. The broken back of your phone is not considered to be a "screen replacement"... I'm pretty sure that's more expensive, but check w/ ubreakifix.
I once broke the back of my galaxy s8+ and had to use my full deductible for a replacement phone w/ Asurion; that was like $200 or so... It's amazing that if you don't have a case on your phone and drop it, you can be out a lot of $$$; even if you do have a great screen protector on the front, because as you say the back is glass too. - - Good luck!