I’d like to suggest that you add the option to tip any amount without upping the order quantity. I had to place two orders and tip x two to get to the amount I wanted to send to the seller. Also, along the same lines, can you make tipping a separate function? I don’t want to tip at the time of the order. I’d rather wait until I receive the final product and then send the seller a tip. That doesn’t seem possible, at the moment, and it’s kind of a pain.
I believe you are talking about tips that sellers add as gig extra. Fiverr has a tip feature just like you describe. After you mark an order as complete, you can send a tip to the seller. You can even pick the amount and the tip will be added after the gig has been completed. The tips you talk about are simply gig extras that sellers have added before the new feature was out. That said, you can tip a buyer after the order has been completed - just don’t pick the Gig Extra if it says I will accept your tip or something like that.