So ive managed to figure it out.
I created a database entry call postage cost. The checkout button forwards you to modal where you select your postage. The buttons have conditions to only display if cart weight is under 1500 and another button if cart weight is over 1501. The user then selects which ever button shows ( kind of like a confirmation of the service ) this updates the postage field in the database to the relevant cost. It then forwards through to the checkout page where you can pull the postage cost from the database again to add to the order total.