It is better for you not to capture the details of the user and that of the tour in the same form,the way you structured your database will play a role here:
Most tourist packages have standard details such as the price per head, departure date and arrival date.  
In integrating any e-payment, it is expected that you show the user what they are paying for (and even allow them confirm it) before you debit them.
Following the above, you could display all the available tour(s) as well as their prices for the user to see. The user would then select the package of interest and proceed to make payment. If they have account, they should login and if you are implementing guest check out, you should display the form that captures their details as well as the amount they are due to pay. Here are suggested interfaces you could use

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