please solve my postage problem

[rabbitnutrition]

i will be setting up a few categories but at the moment i only have the seeds category up and running. i am using a flat rate of 90p for however many packs of seeds are bought whether it be 2 packs or 20 packs. it's working well and i'd like to keep it that way but......
now i want to add listings to other categories without changing the option for the seeds.
i am using royal mail. i think i'll have to go by weight? didn't really want to do it that way but hey-ho if that's the only way..
prices are:
up to
100gs £1.33
250gs £1.72
500gs £2.16
750gs £2.61
1kg £3.15
and so on....
please can someone tell me what i need to do, what i need to delete/disable/enable etc.
i'm new to this

[Ajeh]

If the shipping module that you need to use for everything else is based on weight, the set the weight on the seeds to 0 and customize the shipping module to check if any seeds are in the cart and if so, add the 90p to it ...

And example would be for master_categories_id 12 and 18 to be charged the 90p you can calulate it with:

        $extra_shipping = 0.00;
        global $cart;
        $chk_categories += $_SESSION['cart']->in_cart_check('master_categories_id','12');
        $chk_categories += $_SESSION['cart']->in_cart_check('master_categories_id','18');
        if ($chk_categories > 0) {
          $extra_shipping = 90;
        }

then add the $extra_shipping to the cost in the $method of the shipping module ...

[rabbitnutrition]

thank you
the seeds categories and sub categories master id's are 67, 68, 69, 70 and 71. where do i need to go to change them all to 0 please.
when i have done that, i assume i'll need to use the perweightunit?

[Ajeh]

If you are referring to changing the weight to 0 for these categories that hold products you could use, after backing up your database, the SQL code:

UPDATE products SET products_weight = 0 where master_categories_id == XX;

where XX is your master_categories_id for the Category holding the Products ...

[rabbitnutrition]

sorry i was asking where i could find the code you supplied in your 1st response

[Ajeh]

You would need to put that code in the shipping module that you are using in the function quote ...

Fine where the cost is set in it and add the:
$extra_shipping

to it so that the calculated charge would be added to the cost for the shipping module that you are using ...