So if you use the whole 3GB, Zevvle will cost £2 (monthly sim fee) + £5 (first GB) + £2 (second GB) + £1 (thirds and later GB) + whatever you use for calls and texts = £10+. It’s an extra £3 for unlimited calls and texts, so if you are regularly using more than 100 minutes or texts (any combination of) regularly, then it’s cheaper to use this option.
Do you use exactly 3GB every month? Do you ever go over the 3GB limit, and how much are you charged if you do? With Zevvle in the months that you use less, then you’ll be charged less, use more it’s only £1 per GB. If your usage is very variable, then with Zevvle you’ll get a reasonable based on your actual usage. With bundles it relies on people not using much/all of the bundle for the operators to make money.
With Zevvle the idea is that if the price goes down, it goes down for everyone, rather than some people being able to haggle for a better deal if they phone up or use the chat each year. There are several businesses that I’m aware of who are successful by having a standard price list, and standard discounts due to long term lock-in, rather than aiming for the cheapest pricing to attract customers.