It was next to impossible to find flat cables for Fitbit Charge HR. This is the cheapest I've ever seen them and TBH there's never enough of spare cables to any device.
Enjoying my Prime so I don't have to care about the postage but if you're not having prime, the cheapest option for postage is £1.99 for the "One-Day Amazon Locker Pickup get it tomorrow".
To get the cables for the £3.91 you have to click on the Promotion Message saying "get 25% off".
There's over 150 reviews and 18 months hassle free warranty too.