There is no branding or reference to A2Billing or Star2Billing on the Customer UI, only on the Agent and Admin interfaces, so you are free to modify the Customer UI under the terms of the AGPL.
However, in some cases, there is a requirement to re-brand the application for commercial reasons, e.g. you are selling A2Billing along with other value added extras such as support or carrier services, and one of the conditions of the AGPL is that all copyright, author and AGPL licensing be left intact, and it must also be displayed on any modified version, where ever it was displayed in the original version.
In this instance, a “Permission” can be purchased which will allow you to re-brand the Admin and Agent interfaces which will cost €300 Euro per A2Billing instance. However discounts can be arranged for bulk purchase. Revenue from the sales of permissions will go to help support Star2Billing S.L. who continue to develop A2Billing.
For further information, or to purchase a permission to re-brand, please contact us, or purchase via Paypal below, and we will contact you.
The full text of the license can be viewed at https://github.com/Star2Billing/a2billing/blob/master/COPYING