Prices come from what an order weighs, and what an order weighs comes from your products. This is the one piece of housekeeping that affects every single quote.
Weights are not optional
A product with no weight is quoted as if it weighed nothing. The order still gets a price — just the wrong one, and always too cheap. A $2 quote for a 4 kg parcel is almost always a missing weight.
Weights live on the product in Shopify, where they always have. Set them there, and every app and report that reads weight agrees.
Sizes are optional
Product sizes only start to matter above 5 kg, where a large light parcel can be charged on the space it takes up rather than what it weighs. Below that, sizes change nothing — see Parcels over 5 kg and Do I need to set up packaging?
The quick check
Quote a product you suspect on Test rates (in the app's menu). A suspiciously cheap price is the tell — it means the weight going into the quote is zero.