Hello,

quick question: Your contracts with the customers are managed through the Subscription app or do you have a custom app developped?

Edit:

No need for a custom App, you allready have everything you need in Odoo. This is how I would go about it:

1- Using Studio, create a new Report in the Subscription App (a Report is what pdf files are called in Odoo). You can easily pull fields from your Subscription app and add them to the pdf. 

For a guide on how to create and customize reports, watch this video:

https://www.youtube.com/watch?v=iFghjU_hWMQ&t=60s

2- Create your email template. Head into Settings --> Technical --> Email Templates and create a new one.

In the "Applies To" field, select Subscriptions (i do not have the Subscription App but i will show you a similar thing with Sales Order).

In the body of the email, if you need to pull a field from the Subcription App, just type "/" and select Dynamic Placeholder. and then choose the appropriate field. For example (below), let's say you want to show the customer name, you would do the following: 

/ --> Dynamic Field --> Customer --> Name --> Press Enter , then Odoo will automatically convert your choice into the proper "field name" for the email template. Like so

If you want to attach the PDF Report you create, head over to the Settings tabs, and then in Dynamic Reports field, choose the report you created.

Also, do not forget to set the To (recipients) field in the Email Configuration tabs to "{{ object.partner_id.id }}" (without the "" "") so that it sends it to the actual client for whom the subscription is ending in 3 months.

3- Create an automation that run daily to check the end date of all your subscription and that send the email template when it finds one that expires in 3 months before the end date of the subscription that will send the email template you just created. This you will need someone else to help you with as I honestly do not know how to code it. I am just here to help you with the logic of the process.

Regarding of whether you could have Odoo send the physical mail, there are a few options:

1- You could get to that is either have the email template CCed to someone in your office who will have to actually print or mail it (cheaper option). 

2- You could use a service that offers emails to physical mail, like https://click2mail.com/email-to-mail . Basically you email them the letters you want to send and they physically print and mail them (you will need to create a separate email template as per their instructions i imagine).

3- You could have a partner developp a module for you that connects your Odoo to one of the many "We send physical mail for you" Apis.

P.S: https://docupost.com/ has a Zappier integration that could be a potential solution to your physical email thing.

Hope this helps.