Add online payment process in an existing process

I have a website which provides online questionnaire to people. I want people to pay BEFORE or AFTER they have done the questionnaire. I have already taken online payment services from a provider and have all the details with me. I want an experienced PHP Programmer to develop the following things: 1. Before the user can access the questionnaire page, check if the user needs to pay before the questionnaire (this setting is saved in the db), if yes, get the default currency for that user (saved in the db) and the amount that he should pay (saved in the db). 2. Create a payment record for the user in the db and Present the payment option to the user. 3. Once the payment is done, the system will send payment notification, read the notification and match it with the created payment record (created in step-2) and update the payment record. After updation, present redirect the user to the questionnaire page. 4. The same kind of check and process must be implemented AFTER completion of the questionnaire and before presentation of report. (step-1 and 2 ) The work has to be done on an existing system and I will provide detailed documentation to the selected freelancer.