Thanks for getting back here, @pffmanager. I've come to provide further information about your concern.

Yes, in QuickBooks Desktop (QBDT), when you add the tip as a line item in the invoice, you'll have to set the specific amount of that item. To give you detailed guidance in creating an invoice in QBDT, please check this article: Create an invoice in QuickBooks Desktop.

For your second concern, if you're using the QBDT Enterprise version, you can setup a percentage to pay by creating payment terms. If not, you can split your invoice in half to achieve the 50% payment deposit. For more information on receiving an invoice payment, feel free to visit this link: Record an invoice payment.

You might also want to use the Undeposited Funds account when receiving invoice payments that hold your customer's deposits and combine them later into a single record in QBDT. 

I'm always around in case there's anything else you need about adding tips and setting up a percentage to pay in QBDT. Don't hesitate to leave a reply below. I got your back.