Hello Andrew Brown,
It's hard to say for sure without seeing the checkout screen, but the most likely explanation is the math. Microsoft account balance generally has to cover the total including VAT, not just the sticker price — and at UK 20% VAT, £69.99 would check out at roughly £83.99, which is more than your £75 balance. If that's the case, the store can't apply the balance as the sole payment and falls back to the card for the whole amount.
If the post-tax total is actually within your balance and the card is still being charged, there's a separate pattern people have reported where the checkout screen displays the full amount against the card but only actually takes the difference. I can't promise this is what's happening on your end, but a couple of things have helped others in similar threads:
- Try completing the purchase via the Xbox web store at xbox.com (signed in) instead of the console or Xbox app
- Or switch your backup payment method to PayPal under account.microsoft.com > Payment options, then retry
Also probably worth a quick check that there's no past-due amount on the account under account.microsoft.com > Services & subscriptions, since an outstanding balance can force card use until it's cleared.
If none of those line up, this would be a billing/account question rather than a technical one, so the right next stop is Xbox Support directly.