How to enter amounts in payment fields

Your last form ID https://www.jotform.us/form/90635952203152 is configured with Stripe payment. Are you looking to let people enter a custom amount in the payment field? Or, are you looking to have a quantity box? You already have a quantity dropdown in the first item. You can change this to a textbox in the item settings.

Here's a complete guide for setting up the payment options: https://www.jotform.com/help/225-How-To-Add-Options-In-The-Payment-Form.