How should I sort out next delivery postage

On my website I do not offer next day delivery ..... as it is not always possible on every product and depending on when I check up on my orders. The website just has flat rate postage irrespective of how much customers buy.

This is working fine.

However, I have told customers to get in touch if they want next day delivery as it is sometimes possible on selected items. When a customer gets in touch over this I have asked them to pay as normal and then I send them a further request for payment - to cover the extra postage.

This is not working very well as, sometimes they can't get the paypal request for money to work and also they have to go through the process of paying me twice which is not ideal

Is there some way I can set up another option on the website for special postage....where I can easily alter the amount. When these customers call me I can then get them to put in the special postage option and just pay me all at once instead of messing about.

or does anyone have any better suggestions?

Thanks

The only difficulty is that your offer for Next Day delivery is arbitrary (it's sometimes possible on some products - a fact determined by a human's brain).

No computer in the world can anticipate this availability automatically! So building it into the programming is not possible.

You either offer reliable next-day on all (or appropriate) products, or you stick to your current (manual) system.

If you want Next Day to be offered by the webshop, then setting up an additional shipping option is relatively easy. But you are then committed to offering the service 100% of the time...