Here is the official Medicare Part B information on saline.
https://connect.mayoclinic.org/discussion/7-saline-prescription-and-medicare-part-b-payment-quandry/?pg=1#comment-803092
It's a tough slog to read your way through this, but worthwhile if you want the saline covered in the long term. I believe that the trick might be to get your doc to prescribe a nebulizer, to be secured through Medicare from your pharmacy (I KNOW Walgreens carries the one they cover.) That opens the Part B account, then the scrips are covered through Part B.

Different people make different decisions - some choose to go with GoodRx or pay out of pocket, others push through and get Medicare to cover it. Sometimes you need to change pharmacies - not all have Part B contract with Medicare.

Sue