If I offer my customers a free trial, who pays for their usage during the trial?

You do, unless you build usage into the trial. Whether you include usage is up to you and depends on the niche. For a premium service you may not charge for it. For a 24/7 AI receptionist you can say the trial is usage based and charge per minute. For something like Google review requests sent by text or email, the cost is so low that you can absorb it.

To include usage in a plan, open the SaaS configurator and the SaaS plan. The easy way is to include a credit, for example $10 for everyone, that you expect their volume to fit within. The service-based way is to set an included quantity per service, for example 1,000 emails, and you can choose that they cannot go over it.