You can use the payment page on your website to charge a customer for something that is not in your inventory (for example: extra shipping costs). You can both designate a specific amount or leave it open for someone to enter any amount.
Step One: Go to payment page on your website
Simply type /payment.php after website domain in your browser.
Example: https://www.YOUR_DOMAIN_HERE/payment.php
You can send this URL to a customer and they can enter an amount to pay.
If you want to specify the amount to be paid, simply edit the URL to include the amount as shown in step two below.
Step Two: Add parameters to your URL to set amount to pay
Simply add ?amount=XXX after the URL /payment.php URL from step one.
Example: https://www.YOUR_DOMAIN_HERE/payment.php?amount=12.50
The above URL will allow the customer to pay $12.50 through your website checkout system.
We even use the same page on the Bibliopolis website as shown in the following example.
Example: https://www.bibliopolis.com/payment.php
Example with amount: https://www.bibliopolis.com/payment.php?amount=12.50