What do Compare-at price and Price reduction mean on the Products page?

They are prices from Shopify, shown in the From Shopify section of a variant's page, below the Unit cost section. The Compare-at price is the old price your shop shows crossed out, marked GMV basis. The Price reduction is the gap down to what the variant sells for now, with its share. The section also shows the SKU and the Sales price.

An example: the compare-at price is €80,00 and the sales price is €60,00. The price reduction is €20,00, which is 25,0% of €80,00. On your P&L, the €80,00 then counts in GMV, the full value before any reduction. The €20,00 counts in Price Reductions.

A compare-at price below the sales price is ignored, so it can never make your sales look bigger. The tip beside Sales price says so. With no compare-at price at all, GMV simply counts the sales price.

The Sales price label says how much VAT is inside it, like Sales price, incl. 19% VAT. The summary at the top of the page says the same. The rate comes from this product's own sales over the last 365 days: the rate most of them carried. nouz uses Shopify's own rate where Shopify gives one. Otherwise it works the rate out from sales of €20 or more without VAT, because small sales round too much.

A product that has not sold yet uses your store's usual rate. If your shop's prices exclude tax, no VAT is taken out, and the label says excl. tax. Before your store's first sale the VAT is not known, so the page says VAT not known yet and shows no margin. Every sync checks the rate again.