Sellers remorse is the same as buyers remorse from the other end of the deal. So you sold something finally! Now that you have as you calculate shipping and fees you wonder if it really was a good deal, was it profitable?
The best thing to do is to really try and stand back and look at the whole thing from as much distance as you can realistically achieve. One sale more or less is not going to make or break your business. Look at the basics is your process sound? Is it the best least time consuming least expensive yet professional service you can provide? The kind of service you yourself would want? If the price was reasonable for what you paid and fair to the customer then overall it’s probably good. If shipping and costs ate into the profit on this one item, then it may not have been an item you want to go out of your way to offer. Overall do not get bogged down keep following your process looking back over at least thirty days will show you a more accurate picture overall. Finally just enjoy the moment, keep working toward your goals.