How to do proper sales order on service item

We are a company that advises clients on how to import products from the US to Central America.

we act as an intermediary of purchases

So how we do proper invoices

We have three scenarios:

  1. Customer purchase the product on his own, then the supplier ship to our wharehouse

The only item we need to invoice here is our service. Handling service

  1. Customer request for us to purchase the product
    We purchase the product from the supplier then handle the process until we deliver to the customer

We need to invoice the product from the supplier (There is no profit here)
And our service. Handling service

  1. Customer request to use the product and make an initial 50% down payment on the product

We purchase the product from the supplier using the 50% from us and 50% from the customer. then we handle all the process until delivery

We need to invice the product from the supplier (no profit)
And our service. But we need to record the down payment.

My questions here are:
We need to create Purchase order for each item ordered to supplier in behalf of the customer?
This items are very unique.

How we keep accounting linked?
I mean, our profit margin comes from our services, not from purchasing the products, even if we purchase on behalf of the customer using 100% of our money.

Out to get profit margin from our service?
Our service is per item weight
Our supplier charge us $1 per lb, We charge $3 per lb to our customer
I think in our sales order we need to “sale” this item per lb.

We need to get our profit margin

Best Regards

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