A2X has taken an average/blended cost model for our cogs function. Cost prices can be updated as often as you need and those average costs will be applied from that moment forward. Some people update their average costs on a per settlement or monthly basis.
Currently A2X is primarily focused on the financial transactions, sales, fees, charge-backs, reimbursements etc. - COGS is a function of sales, its the cost price of the sales you made. However as you would know using a dynamic cost model, a number of inventory transactions can happen at different times and involve FIFO/LIFO situations etc and these can all have an impact on your accounting too. For example when a return comes back into stock sell-able, or when Amazon damages a unit.
Today those inventory transactions are typically captured by accountants in a the standard inventory accounting approach:
Opening stock + purchases - COGS - adjustment = closing stock.
Our feedback from users was that when users out-grow simple costing, they typically move to one sort of advanced inventory tool to manage vendors, purchase orders and costing schemes (LIFO, FIFO etc) separately from their accounting system.