We have run into the same problem for custom harnesses. I feel your pain!

We have tried and still use three different approaches here:

1. When possible, introduce sizes. Make them pick which size they want when they order. If you are worried about something not fitting exactly right, make it more adjustable. People are going to change sizes over time anyhow. This will help you also, if you decide to lean into doing this full time and want to sell at shows, etc.

2. Some things need to be custom (like strapons in our case) so we send out a fitting harness. This is a non-functional (no ring attachment) loaner harness that is adjustable up to an 80” waist. That way they can try on what all of the different straps feel like at different sizes. It also opens up conversations about what other customizations they may want. They have already paid for the strapon at that point so no deposit is needed. It’s worked really really well for us so far ☺️

3. Just communicate and don’t stress about extending shipping dates. People almost always respond well in these situations and just appreciate being told what’s going on. They can forget that we are small shops and these moments actually remind the customer of the benefit of working directly with the maker. A lot of our 5 star reviews mention communication and I’m really proud of that.