At any time, you can select the "$" icon in your dashboard to link your Stripe account and enable payments for your app in AppCenter. However, this change will only apply to your app when a new release is submitted; to link a new account or unlink an account you will need to create a new release and submit it for publishing.
Monetizing your app allows you to accept payments from users when they download your app. AppCenter displays the suggested price as the download button, along with a dropdown where users can choose their own price.
Even if you want to give your app away for free, you should consider monetizing and setting the suggested price to $0. This lists your app as free throughout AppCenter, but allows users to send you optional payments when installing and from your app's listing.
Each payment is split between you and elementary 70/30, with a minimum payment to elementary of $0.50 to cover distribution and payment processing. For example: with a $10 payment, $3 is paid to elementary and $7 is paid to you. For a $1 payment, $0.50 is paid to elementary to cover distribution and payment processing while $0.50 is paid to you.
There are no enrollment fees or subscription costs to publish your app in AppCenter.