Payment details is where you should add information such as Bank Details, payment method etc. While this field is intended for bank details, you can provide any information in this field you like.
To get paid online you can upload payment Barcode/QR code image or insert payment URL/link. The uploaded barcode/QR code image will appear on Invoice PDF and your client can scan that Barcode/QR code and pay you online. After inserting payment URL/link it will create a create a clickable Pay Now button with a clickable link on the PDF. Due to which you would get paid online.
Change the label/Text of Grand Total and Adjustments as per your choice. Adjustments helps you to round off the Grand Total. Add any +ve or -ve charges that need to be applied to adjust the Grand Total Eg. +10 or -10.