Variable Subscription is an extension of the Payment Button for PayPal WordPress plugin (also known as WP PayPal or WordPress PayPal) that can be used to add a recurring price field to your PayPal Subscription payment buttons. This lets buyers set the amount they will pay for your subscription products. It can also be used for accepting recurring donation payments.
In order to add a price field you can use the following shortcode.
[wp_paypal button="subscribe" name="My product" a3="" p3="1" t3="M" src="1"]
Leaving the a3 parameter empty implies that you wish to allow buyers to enter a custom amount for this item.
When the button is rendered on the front end, a price box will appear with it.