Yes, you can customize the booking flow, but what you can change depends on the plan you choose. Most white label portals let you decide the sequence of steps your customers see, from searching for flights or hotels through to payment and confirmation. You might want to show payment options early, or you might prefer to collect customer details first before showing prices. The flexibility you get is part of what makes a white label solution different from just using someone else's booking site.
The exact customization options vary depending on which features are included in your plan. Some plans give you more control over the order of screens and which information you ask for at each stage. Other plans have a fixed flow that works well for most travel businesses but leaves less room to change things. When you are thinking about which plan suits you, it helps to write down the booking journey you want your customers to follow, then check whether your plan supports it.
You should also think about what your customers expect. A complicated or unusual booking flow can confuse people and make them abandon their booking before they finish. The best customizations are usually the ones that make the booking faster and clearer, not the ones that add extra steps. Talk to your account team about what customization options are available on your plan and what other travel agencies have found works well in practice.