I'm totally new to odoo, set up my own instance (version 15.0), everything seems to work well, I can use the web interface, install apps, etc, etc.

First, I'd like to evaluate whether I really need odoo or not, these are my circumstances/requirements:

1. I'm selling a single physical product online, the product comes in about 10 variations (size) each with its own price.

2. There are various platforms on which I'm selling the same product (my own website using a home grown ecommerce solution, etsy, etc.) The price varies with the platform. And the costs (fees) associated with the given platform also varies.

3. Currently when an order comes in from any of the platforms, I get an email with the product variation, price paid, email, name and shipping address. Customer also gets a confirmation email.

4. I enter the email, name, shipping address, price paid, fees taken and product variation into a google sheet by hand.

5. Edit a shipping label by hand and print it.

6. Stick the shipping label on box, take it to post office and mail it.

7. Enter shipping cost and tracking number into google sheet by hand.

8. Send an email to customer with tracking information in gmail (so mostly by hand).

9. After some days email customer with some further info in gmail (so again mostly by hand).

This is basically the life cycle of an order. In the end everything is in a google sheet document and now I have all sorts of tables for aggregated monthly sales, revenue, profit, in two different currencies. Google sheet is useful for converting from one currency to the other using the date of the order for looking up the exchange rate.

10. Using the aggregated data from google sheet I'm creating nice plots on various sales metrics.

Now I'd like to automate the above as much as possible with odoo.

But I wouldn't want to use a new ecommerce platform, so will not use odoo's website builder at all, but would like to stick to what I have already in terms of website and payment processing. So let's start with 3. above: I have all the necessary information for an order programmatically and would like to use odoo to record this order.

Is there an odoo API I could use from php to record an order? If I have to enter the order by hand I'm back to square one.

I've been checking out the Website app, it has a nice analytics page, list of orders, unpaid orders, etc. This is cool, but what I'm not sure about is if all of this is only usable once I actually use the odoo website builder and sell there, or can I use these features with an external ecommerce solution and simply manage the orders here? And enter new orders programmatically through an API as well as enter orders manually? 

Is it possible to simply enter all of my previous customers via an API?

Is it possible to extend the "orders" web interface for me to enter tracking information? (I don't have the tracking number programmatically, I get this at the post office so have to enter it manually.) And can I add two extra buttons next to each order, first of which would send an email to the customer with the tracking number and the second button would send an email with some further information?

Basically, all I need is a customizable "order management system" and probably I'll be only using about 2% of odoo or less. But that's okay if it makes my life easier :)

A must have for that is a php API and I'm not sure how much that is a focus for odoo, hence this very long question :)