There is an option to add a tip to an order on the payment page when a customer checks out on Shopify.
How it works in Veeqo?
Veeqo creates a product with title 'Tip' once first tip is imported with an order;
All further tips will be linked to that exact product;
'Tip' product has infinity stock;
Amount of each tip will be in order line item;
You can use 'Tip' product in your further reports in Veeqo.