Hi,
If you are on the Flat Rate Scheme or considering to join the scheme then you will still charge the same liability to your customers, depending on the products ie 0% or 20%.
You then apply the percentage based on your chosen trade sector against the gross sales and pay this over to HMRC.
If you are providing a lot of zero rated sales then it probably wouldn't be in your interrest to join the Flat Rate Scheme as you could be losing money.
Please see the guidance below:
Basics of the Flat Rate Scheme
Determining your flat rate turnover
Thankyou.