Is there a way to allow the user to edit a custom field on a line item at checkout?

[bandini]

So, let's image I sell t-shirts and I want the user to be able to, at checkout, edit a custom field on a single line item. A text he would like at the back of his tshirt, for instance. 

Is there a way of doing that? I was trying with a cart transformer function, but I don't think that's the way

[bandini]

There's none. I'm working locally on this idea.

[bandini]

Yes!

[bandini]

Oh, that I do have.

But it's a fresh store, There's nothing in there. I'm just trying to understand if this is possible and, if it is, what's the best way.

I'm thinking a UI Extension