Can my customers pay by bank transfer or in cash instead of by card on my portal?

Yes, you can usually accept more than card payment, but what you are able to offer depends on the payment gateway you connect and on the supplier behind each booking.

Card payment is the normal default because the money is confirmed straight away, which lets the booking go through to the airline or hotel without anyone having to check anything by hand. If you want to accept a bank transfer, or let a trusted customer pay at your office, the portal can be set up to take the booking and hold it as unpaid until your staff mark the payment as received. Your team then confirms it from the back office, and the customer gets the confirmation only at that point.

The honest limit here is time. Many airline fares must be paid and ticketed within a short window, and some hotel rates are held only for a little while before the supplier releases them, so a booking waiting on a transfer can expire before the money arrives. That approach works best for quotes, group requests and hotel stays with a later cancellation deadline, rather than for cheap fares that have to be issued immediately. Whether a particular supplier allows any delay is decided by that supplier, not by your portal.

The next thing to check is which payment methods your chosen gateway supports in your country, and then agree with your team which bookings are allowed to be held as unpaid and who is permitted to mark them as settled.