Azure marketplace offer questions

How can I automate billing based on user count under a flat fee pricing model(or any other pricing model in that I can send the usage)?

Currently, the billing is calculated manually at the end of each month by multiplying the total number of users added to the application by €19 per user. For instance, if there are 15 users, the total invoice amount would be:
15 × 19 = €285
This amount is then billed to the subscriber.

I want to automate this process. How can I achieve it?
FYI: I'm a SaaS application developer

Additional Details

Azure Marketplace Offer Setup and Questions

I’ve configured an Azure Marketplace offer using a flat fee pricing model with a dimension to calculate billing based on the user count. The payment schedule is monthly. Here's an example scenario:

The dimension is set to €19 per user.
On January 1st, the SaaS application has 10 users.
On January 15th, the user count increases to 12.
or vice versa on January 18 user count decrease to 8 users

Additionally, I have created two plans: 
A 1-month free trial plan. 
A monthly paid plan (as described above).

Questions:
When should I send the user count (usage)? Should it be reported daily or at the end of the month? 
If I report usage daily, will the charges be calculated for each day's usage? 
For example, if I send a daily usage report of 10 users for the entire month, will the calculation be: 
10users} times 31 = 310€  
310  19€ = 5890
Is it mandatory to send usage data daily, or can I report it once a month? 
Where can I view the total usage data reported so far in the Azure Portal? 
Can I send the user count directly as usage? 
If a user switches from the free trial plan to the paid plan, do they need to wait until the free subscription ends? 

Please provide assistance on this