UK Domestic Postage Calculator/Module for Open Cart

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Ajay K.
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Experience Level: Intermediate
We already have an open cart shop, and trade on Amazon and eBay based on a Free Postage pricing model. Everything we sell goes through Excel and gives us our absolute bottom price based on this model. The problem is, is leads to 95% of sales being single line – That is, each customer only buys a single item. The Free postage model encourages this.

In February we are launching a new open cart store where our entire range will be sold excluding postage. We have over 2000 customers a month shopping with us on eBay and Amazon, and we will be primarily marketing this to them. That said, we’ll also look to integrate feeds so that we can pick up google traffic.

Now, the postage problem is a bit of a nightmare. We do not want to introduce a one or two size fits all postage charge, and we will not be offering any free postage regardless of basket size. We want to customise the postage charge based on the items weight and postage format. Postage format is important as our main postage provider, Royal Mail, charges one set of rates for “Large Letter”, and another for “Small Parcels”. 80% of our sales are currently large letter. We sell small things.

Since you may be familiar with Royal Mail, I won’t go into too much detail, but will include a list of prices which also shows the sizes.

We also use also use other parcel carriers, and the rules I feel are too complicated to get the computer to decide.

Currently open cart has a royal mail options but it only determins service by weight. The problem is, royal mail also have strict size requirements. Add to this, 80% of our items are in the cheapest large letter rate, it is clear that this system will not work, unless we have flat rate pricing. We don’t want this.

So we typically price postage on:

Size of Shipped Package?
- We look at the packaged item, and workout what packaging material is required.
- Selection: Envelope; Standard Jiffy; Large Jiffy; Small Box; Medium Box; Large Box

Service to Use?
- Most mail goes Royal Mail. If the item is large letter and the COST price is £4+, we add signed for. If item is small parcel and COST price if £4+, we send with Hermes as it is tracked. All medium parcels go with Hermes.
- Selection: Royal Mail; Royal Mail Signed For; Hermes

Packet Size/Weight?
- Based on the size and weight, we put each product into one of the following postal rates.
- Selection: Large Letter <100g; Large Letter <250g; Large Letter <500g; Large Letter <750g; Small Parcel <1kg; Small Parcel <2kg; Hermes <1kg; Hermes <2kg; Hermes <5kg

Now for a human to work out, it is easy, but a computer is another matter. And when we get to multiple items, it becomes harder.

So, having sat down this evening to design a system for UK Domestic postage (we don’t plan to ship internationally at the moment), here is what I have come up with.


We assign each product with a packet size value. Called points. The system then uses the total across the order along with the total weight of the order to determine the postage charge. We put this in a table, so a simple DB query can look up the correct row.

See the attachment for what this table looks like, and other information on how the value is created.

It may be prudent to group the table into LOCAL; UK Mainland; Europe, just to be future proof. We will probably only use UK Mainland initially.

We could add the packet size value to the new tab you recently made for me.

Let me know what you think.


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