How to show only one payment form, which was selected in booking form by visitor?

This instruction apply only to updates 8.6 or newer in Booking Calendar Business Small or higher versions.

You can use in the booking form at the Booking > Settings > Form page this shortcode for ability to select payment method that will be showing after booking process:

<p>Select payment method:<br/>
  [select payment-method "All payment methods@@" "Stripe@@stripe_v3" "PayPal@@paypal" "Authorize.Net@@authorizenet" "Sage Pay@@sage" "Bank Transfer@@bank_transfer" "Pay in Cash@@pay_cash" "iPay88@@ipay88" "iDEAL@@ideal"]
</p>

Of course, you can remove from the above shortcode, the payment systems, that you have not activated at the Booking > Settings > Payment page, and use only active payment forms, similar to this:

<p>Select payment method:<br/>
  [select payment-method "All payment methods@@" "Stripe@@stripe_v3" "PayPal@@paypal"]
</p>

P.S. This solution was suggested by "Dan Brown". Thank you.