The TS about custom dev is here.
https://hostbillapp.com/license/section 7.
License for Updates and additional Modules:
The Licensee has the right to expand the given License through the purchase of additional Modules or Software Updates.
If the Licensee purchases the “Software Development Kit (SDK)” License, specifically detailed in the Service, the Licensee has the right to create additions/modules to the Software solely for personal use.
The Licensee can purchase the “Developer Edition” Module License, and by doing so acquire the right to use the Modules of the develop version. The Licensee gains access to the Module source code in this manner, simultaneously acquiring the right to further develop or change the Module source code, with the following restrictions:
a. The “Developer Edition” can be run on only one copy of the Software,
b. The Licensee does not have the right to further distribute or sell part or all of the Module code,
c. The Licensee does not have the right to remove license records in the Module code,
d. The Licensee has access to the current Module source code version for only one year. After the passing of the time period for the duration of Subscription, the Licensee can extend the “Developer Edition” Module License an unlimited number of times, otherwise the License expires.
e. The Licensee only has access to the Module version he purchased. A newer Module version, denoted by a subsequent digit, may be acquired by the Licensee under the terms of the License Agreement.
There is feature request as well
https://hostbillapp.com/feature-request/Also pay them to do it.
https://hostbillapp.com/services/custom-development/