Shipping by container ship is already pretty cheap, and of course shipping isn't itself a good that people care about, it's a way to get some other good. It also seems like the supply bottleneck for shipping per se is probably more "number of ships" or "capacity of ports and big canals" than "fuel costs."

We should of course assume that ceteris paribus lower shipping costs produce more shipping on the margin, but I don't see a lot of reason to believe that this is the major supply bottleneck for most goods. I wouldn't expect to see a dramatic change in demand for shipping as a result of 20% lower fuel costs.

Jevon’s paradox?

https://en.m.wikipedia.org/wiki/Jevons_paradox

I believe that generally speaking, shipping costs typically are considered a relatively small portion of overall costs for any given product - just going off discussions in Box by Levinson, in which he asserts that one of the paradigm shifts enabled by containerization which allows for globalization to emerge as a phenomenon is that container-based shipping drops the cost of shipping to a negligible fraction of the product being shipped.

Another thing to consider is that the cost to the shipping company goes down, which might just mean they get more margin to keep for themselves, if all the companies do that and don’t undercut each other.

Naively yes, it will be reduced, because these sails are not free.