Presuming you need to record all your purchase transactions with every single detail, including your supplier data, and if you want to make the purchase/buy menu requires Supplier Information, you can set it to default, so the transaction can not be made without Supplier information. For detailed steps, you can check the explanation below.
Step 1. Go to Setup, Click More Menu, then Select Module
Step 2. Go to Buy Tab, Set Supplier to Required, then Click Save
Step 3. Go to Buy, Try to Make Purchase Order without Supplier
When the setting has been activated, then every purchase transaction will always require a supplier contact, you can check the picture below if the setting is activated & supplier contact is not entered, a notification will appear at the bottom right of the screen.
After the save was made, you are no longer allowed to use the Buy menu without inputting the Supplier Information.
For COGS calculation in DealPOS, we have several calculation methods, including Average, LastOrder, and Manual. To set up, the COGS Calculation, go check this article: Setting Up Product's COGS Calculation
If you want to apply the purchase method with 2 steps of verification, you can do it at DealPOS by configuring the setup, you can check the detailed explanation in this article: How to Activate Approval when using Buy (2 steps)
To change the default payment to Later on the BUY menu, you can check this article for the setup: How to Select Default Payment in Buy