When a customer checks out, Shopify adds the weight of your store's default package to the weight of the products in the cart. That total decides which weight-based rate from Zapiet - Rates by Distance appears at checkout.
Your rates in the app still work as you configured them. If your default package is heavy, a light order can land in a higher weight range than expected.
If a weight-based rate at checkout looks too high for the order, check your default package weight in Shopify before you change your rates.
How package weight affects your rates
Shopify works out the order weight like this:
product weight + default package weight = order weight
The rate whose weight range includes that order weight is the one your customer sees. Price-based rates are not affected.
Example
You have two weight-based rates:
Local delivery: 0 to 5 kg, £9.99
Heavy items: 5.01 to 30 kg, £24.99
Your store's default package weighs 2 kg, and a customer orders a product that weighs 4 kg.
Shopify uses 4 kg + 2 kg = 6 kg, so the customer sees Heavy items at £24.99 instead of Local delivery at £9.99.
Check your default package weight
In Shopify admin, go to Settings > Shipping and delivery.
In the Packages section, find the package marked Store default.
Note its weight.
Add that weight to the product weight in your test cart. If the total falls inside the range of the rate you saw at checkout, the package weight explains the result.
Fix a rate that doesn't match
Pick the fix that matches how you pack your orders:
If the default package weight doesn't reflect your real packaging, update its weight in Shopify, or make a lighter package your store default.
If the weight is correct, edit the weight ranges of your rates so each range includes the packaging.
After any change, test the rate again in an incognito window. Shopify can show a cached rate when the same cart and address are used.
Limitations
Zapiet - Rates by Distance can't see your package weight, so the app can't adjust for it. [Shopify limitation]
If you're still not seeing the rate you expect, contact our support team and we'd be happy to help.