I made a quick edit - I only have propane for my stove so it's a 50 gallon tank, maybe that's part of it?

https://imgur.com/a/GBR3QAP

Actually, this is just for my 50-gallon hooked up to my stove, I heat with oil. If you're heating with a 500-gallon tank maybe it's a different rate?

I just scrambled to put in an order at that rate and here's what I'm getting in Plymouth:

Price Per Gallon $5.7990