How do I see how much I actually made each month?

Open P&L, below Stats in the sidebar. It lists every month from newest to oldest with its net profit: the current month is marked In progress, and each year's header adds up the net profit of its finished months.

Tap a month to see how its net profit is worked out:

Revenue (excl. tax): revenue after every discount, less tax, counted on the day the goods were handed over.
Product cost: the cost of what sold, from the costs on your products.
Processing fees (est.): the fees estimated from your payment methods' rates.
Forfeited deposits: deposits you kept when a pre-order was cancelled.
Fixed expenses and One-off expenses: what you recorded for that month — see “How do I record rent, payroll, and other expenses?”

Up to three charts follow: net profit month by month, each fixed expense's share, and the fixed expenses trend. The two trend charts cover at most the 12 months up to the month you opened, and appear only once there are at least two months to compare, counted from the month of your first sale or expense. The share chart needs a fixed expense that month; until there is one, that section shows Add Fixed Expense instead. On Stats, the last line of the profit card jumps straight to P&L: for a period that is exactly one month it shows that month's net profit, like “October net profit NT$12,300”; for any other period it reads View monthly P&L.

Net profit is operating profit before income tax, worked out from what DingPOS has recorded — not a tax return. If products sold without a cost, the month still shows a net profit, marked “Excludes the cost of N products”; tap it to see which ones. P&L is on every plan. With staff accounts, opening it takes the P&L & expenses permission, which Managers and Staff need approval for by default, because it shows product costs and payroll.