Does the customer pay for the extra parcels?

No. The split happens after checkout, so what the customer paid stays the same. If you want customers to pay per parcel, that has to be built into your shipping rates.

Can I use a weight limit together with other rules?

Yes. Rules that separate items — for example by vendor — run first, and the weight limit is then applied to every resulting shipment.

Yes. On the order page, the Unfulfilled section's menu has Merge fulfillment, which puts items from the same location back into one fulfillment.

Do I need to set weights for every product?

Only for products that can end up in an over-limit shipment. If one is missing a weight, SplitFlow leaves that shipment alone and tells you which product to fix.

How to split Shopify orders by weight

Carriers cap how heavy one parcel can be — often 20, 25 or 30 kg. When an order goes over that, it has to leave in several parcels. Shopify keeps all unfulfilled items from one location in a single fulfillment, so heavy orders end up being split by hand.

Short answer: Shopify does not split orders by weight on its own. You can split a fulfillment manually on the order page, or set a shipment weight limit in SplitFlow so every new order over the limit is split into shipments that each fit.

Watch: a 20 kg limit splits a heavy order into parcels that fit (30 seconds).

Option 1: split one order by hand (built into Shopify)

In your Shopify admin, go to Orders and open the order.
In the Unfulfilled section, open the ⋯ menu and click Split fulfillment.
Choose the items (and quantities) that should go into the new fulfillment, then click Split fulfillment.

This works on the desktop admin or Shopify on the web, not in the Shopify mobile app. If you split by mistake, the same menu has Merge fulfillment to put items from the same location back together.

You do the weight maths yourself. That's fine for a few heavy orders a week, but it gets slow and error-prone at volume.

Option 2: split every order automatically with a shipment limit

Install SplitFlow from the Shopify App Store.
In the app, create a rule and choose the rule type Limit shipment size — or click the Carrier weight limit preset.
Enter your limit, for example 20 kg. Kilograms, grams, pounds and ounces all work.
Click Preview latest order to see what the rule would do. Nothing is changed by a preview.
Save the rule.

From then on every new order that's over the limit is split into shipments that each fit, usually within a minute of the order coming in. To also apply it to recent unfulfilled orders, use Apply rules to recent orders.

How the split is worked out

SplitFlow fills each shipment up to the limit, heaviest items first, so the order goes out in as few parcels as practical. Units of the same product can be spread over several shipments — four 8 kg boxes under a 20 kg limit become two shipments of two. It uses each variant's weight from Shopify (product → variant → Shipping → Weight), so make sure your heavy products have one.

What it won't do

If an item in an over-limit shipment has no weight in Shopify, that shipment is left as it is and the app tells you which product needs a weight.
If a single unit is heavier than the limit on its own, it can't be split further, so that shipment is left unchanged and flagged too.
It only reorganises fulfillment. Order totals, payment status, inventory and what the customer paid for shipping stay exactly as they were.
It works on fulfillments from your own locations, not on orders that a third-party fulfillment service (3PL app) handles.

The same rule type also limits items per shipment — the Max items per box preset sets 5 items per shipment, for example.

Questions

Does the customer pay for the extra parcels?: No. The split happens after checkout, so what the customer paid stays the same. If you want customers to pay per parcel, that has to be built into your shipping rates.
Can I use a weight limit together with other rules?: Yes. Rules that separate items — for example by vendor — run first, and the weight limit is then applied to every resulting shipment.
Can I undo a split?: Yes. On the order page, the Unfulfilled section's menu has Merge fulfillment, which puts items from the same location back into one fulfillment.
Do I need to set weights for every product?: Only for products that can end up in an over-limit shipment. If one is missing a weight, SplitFlow leaves that shipment alone and tells you which product to fix.