Let's start with some tax accounting basics before we address your many questions.
The type of business that you are operating required an inventory for tax purposes, prior to enactment of the Tax Cuts and Jobs Act of 2017. Now, if your business averages under $25 million in gross receipts, you may account for your inventory conforming to your method of accounting. If, as I am willing to assume, you are a cash method taxpayer, you can now account for inventoriable items using the cash method, which means you will have no inventory for tax purposes. You will treat purchases of goods for retail or materials and supplies that are significant components of the goods you make for sale, as expenses at the time you pay for your purchases.
In light of the above, I will try to answer your numbered questions:
1. Cost of Purchases are goods you buy for resale, without performing any significant work on them. Materials and supplies are the things you but that are significant inputs into the process of making things you sell.
2. Incidental materials and supplies, things used in such small amounts per unit of production or which benefit the entire production process, are not part of Cost of Goods Sold, but are reported on Schedule C, line 22.
3. Irrelevant, since TCJA now allows you to expenses everything as purchased.
4. You will not be reporting an inventory. Use Sch. C, Part III, to report your Purchases and Materials & Supplies, but report zero (0) ending inventory, so all costs get expensed.
5. No. That line would be labeling, packaging, etc. Postage is an expense in Part V and carried to line 27a.
