We're still working on an automatic way of doing this, but in the meantime we've got a couple of great ways you can use to pass the fee on to your client using features Joist already has.
One easy way is to use Markup, which allows you to build the fee into the estimate or invoice invisibly. Just add a markup of 3% and your credit card transaction costs will be covered.
Another way is to add a line item to your estimate or invoice, which you can call "credit card fee" or "convenience fee" or anything you feel will communicate the meaning to your client. Then just calculate what 3% of your total is and make that the cost of the item.
Many clients are willing to pay this fee for the added convenience of paying with their credit card, and many Joist contractors successfully pass the fee onto their client using one of the two options listed above.