how to calculate the courier rate based on weight and postcode?

it's a pretty simple collection pipeline,

$packageWeight = 2.1;

$courierRates = [
    [ 'weight' => 1, 'cost' => 8.00 ],
    [ 'weight' => 2, 'cost' => 9.10 ],
    [ 'weight' => 3, 'cost' => 11.10 ],
    [ 'weight' => 4, 'cost' => 13.10 ],
    [ 'weight' => 5, 'cost' => 15.10 ],
];

$rate = collect($courierRates)
    ->sortBy('weight') // optional: ensures the rates are in weight order
    ->first(fn($rate) =>  $rate['weight'] >= $packageWeight);

$rate['cost']; // = 11.10

Where you store that data is entirely up to you, if it makes sense to have it in a model/DB you can. or you can create your own simple courier class /factory whatever you need, with the rates hardcoded (not ideal) or loaded from a file, csv, json whatever.

so you could do

$packageWeight = 1.1;
$destination = 'b';

$courierRates = [
    [ 'weight' => 1, 'cost' => 8.00, 'location' => 'a' ],
    [ 'weight' => 1, 'cost' => 10.00, 'location' => 'b' ],
    [ 'weight' => 2, 'cost' => 9.10, 'location' => 'a' ],
    [ 'weight' => 2, 'cost' => 11.10, 'location' => 'b' ],
	...
];

$rate = collect($courierRates)
	->filter(fn($rate) => $rate['location'] == $destination)
    ->sortBy('weight') // optional: ensures the rates are in weight order
    ->first(fn($rate) =>  $rate['weight'] >= $packageWeight);

$rate['cost']; // = 11.10

or if you've got eloquent models for the couriers, rates & locations you could do something like this.

$shippingCosts = Courier::with('rates.location')->get()
    ->mapWithKeys(function($courier) use ($destination, $weight) {
        return [ 
            $courier->name => $courier->rates
                                        ->filter(fn($rate) => $rate->location->name == $destination)
                                        ->sort('weight')
                                        ->first(fn($rate) => $rate->weigth >= $weight)->cost
        ];
    });

$shippingCosts == [
    'Courier A' => 3.00,
    'Courier B' => 3.00,
    'Courier C' => 3.00,
];

N.B. that's also a pretty bad eloquent query you'd probably want to query / scope it by the location so you only load the relevant rates not all of them if you have hundreds..

class Rate extends Model
{
    protected $fillable = [
        'courier_id',
        'weight',
        'cost',
        'location_id',
    ];

    public function location()
    {
        return $this->belongsTo(Location::class);
    }

    public function courier()
    {
        return $this->belongsTo(Courier::class);
    }
}

class Courier extends Model
{
    public function rates()
    {
        return $this->hasMany(Rate::class);
    }

    public function locations()
    {
        return $this->hasManyThrough(Location::class, Rate::class);
    }
}

class Location extends Model
{

    protected $fillable = [
        'name',
    ];
}

At a medium sized trucking and logistics company I worked for we had rate tables, lookup tables.

This was a while back in my Java days. We basically had a series of look up tables where you know that a load of pecans (example) would be haulded by a certain carrier for x amount per pound or ton, etc. But yes set up some basic relationships to help you. Surely each carrier has various rates I'm guessing. And of course length to the destination must be considered.

Remember also many larger carriers have apis you can tap into or updated rates you can download and import one of the other.

Another thing to consider is negotiated rates that remain stable x amount of time.

@nickywan123 What is it you’re actually asking? If you know the weight of your parcel and the postcode it’s going to, and you query individual couriers with this information to get prices for that weight and postcode, what is it you’re actually struggling with?