Can I stop calculating inventory?

Yes, the post from @Anonymous_ is accurate.

The Cost of Goods Sold formula for retail businesses, generally, is:

Beginning Inventory (at the beginning of the tax year) Plus Purchases Minus Ending Inventory (at the end of the tax year). In the instance you cited, that would be $3012 (beginning inventory) less $2925 (ending inventory) which would equal a CGS of $87 (since you apparently had no purchases during the tax year).

In future years, you can enter the costs of materials and supplies used to produce your books or, alternatively, enter those costs as purchases.

Note that, if you are a small business taxpayer, you are not required to keep an inventory for federal income tax purposes.

From Pub 538: Exception for Small Business Taxpayers If you are a small business taxpayer (defined below), you can choose not to keep an inventory, but you must still use a method of accounting for inventory that clearly reflects income. A small business taxpayer can account for inventory by (a) treating the inventory as non-incidental materials and supplies, or (b) conforming to its treatment of inventory in an applicable financial statement (as defined in section 451(b)(3)). If it does not have an applicable financial statement, it can use the method of accounting used in its books and records prepared according to its accounting procedures. See Regulations section 1.471-1(b). If, however, you choose to keep an inventory, you generally must use an accrual method of accounting and value the inventory each year to determine your cost of goods sold.