So we need to increase our postage costs for seasonal times of the year such as Christmas; but it doesn't seem to be possible with shopify, and bizarrely there doesn't seem to be any apps that support this?
It's very common for people to pick a delivery slot on perishable goods such as food - and these slots become expensive during peek times such as Christmas.
Does anyone know how we can achieve this with Shopify?
Holidays are different from one country to another country. There for it would be a localised postage you may need to guarantee deliveries advanced postage if possible. This is a common problem solutions for one country do not necessarily solve the same holiday season in others. I hope this a bit