We're currently hard at work on a mechanism to adding tips to sales/purchases, but in the meantime, we've got an easy way to add a tip to a customer's purchase.
Step 1 - Create a Tip Product
Create a new product called simply, "Tip". You can choose whatever category you'd like for this product, but don't set a drop in price. This is the first step in creating a series of tip packages. Setup your Tip product to match the choices shown in the screenshot below. Note that you only have to make this product one time, not every time you'd like to make a purchase.
Step 2 - Create a Tip Package
Create a Tip package to match the amount of the tip you'd like to charge the customer. For example, if you wanted to charge a $25 tip, you would call the package "$25 Tip". Set the price as $25, the sessions to '1', and the expiration date to '1 day'. Follow the choices to match the screenshot below. Note that you only have to make this package one time, not every time you'd like to make a purchase.
Step 3 - Making the Purchase
When making the purchase for the customer, head to the 'Customers' section of your business account. Search for the customer and the hit "Purchase" link on the right side of the row. Then go to the $25 Tip product (or the name of the tip product you created), hit Purchase to charge the customer.
You can create as many Tip packages as you'd like, just set them to 'Business Use Only' and they won't show up in the list of packages that your clients see. Only you as a business (or your employees with access) will be able to add the tip onto the purchase. If you'd like to have the client also purchase the tip option, turn the 'Business Use Only' option to off, then they can make the purchase!