Like most other things the answer is "It depends..." Different suppliers and areas are different.  Historically having enough storage to hold all you'll use in a winter and buying in the summer at a lower price has worked best. The big advantage besides price is having it on hand in the event there is a shortage, which tends to happen at least once a season when things get cold for extended periods.

The disadvantages are you have a big expense at one time, you have a risk of loss if there is a mechanical failure, and the cost of the tank(s)  For years and years I was buying used tanks for $1/gallon or less. (We have 13 locations on the farm with grain dryers, irrigation engines, shop, houses, etc).  The last couple years I've seen tank prices jump as high as $5/gallon.  At that price you have to ponder on the value.

I am a seasonal propane delivery driver.  My company has various plans the customer can use.  Our usual way (assuming your credit is OK) is net 30 with a cash discount if paid within 10 days  A budget plan spreads the cost out over the year. We also take credit cards.  Not sure if they are set up for ACH yet.  It will happen eventually.

We have 2 different contracts.  One is pre-pay for so many gallons at a set price.  The other is pay 10 cents per gallon (based on historical usage) and lock in the price all winter regardless of number of gallons.

Other companies have variations.  Generally the more aggressive a company is at trying to gain business the better the deal for the customer.  Just make sure the deal isn't better than it sounds.

I've been driving since 1992.  I've only seen one or two times forward contracting didn't at least break even if it didn't save money.  A couple odd years the customer would have been better of without a contract.  But it's unusual.