What exactly do you mean by "dynamically" purchasing these vouchers?
I don't know, you tell me. I just wanted to understand the way you purchase these vouchers. So the traditional way I know is that you receive a file with the codes. However, I worked with Amazon vouchers the last time more than 10 years ago (where I work now Amazon did not exist until recently and they also have a very limited offer). So maybe there is an API to directly purchase those vouchers individually via this API. But I guess not, so you can forget about this.
Now, as you tell me that you have an open survey, which anyone can access, I would highly discourage you to send vouchers automatically. There are several problems and risks with it:
- People will abuse it. People will fill in your form just to get to the incentive. People will find ways to automate this and will create as many responses as possible. I don't know the value of your Amazon vouchers, but it probably will have some value and even for 5 or even 1 Dollar per answer, people will try to make money with it.
- Even if there are no abusers, with an open survey, there is no way you can control who accesses your survey. Word on the internet spreads quickly, when there is an incentive involved. Depending on your survey this can have two effects: you get a lot of responses that do not make any sense for you (e.g. not your target group). Another thing is your budget. If you leave this open, it can grow quickly to a considerable sum.
In general, in market research for example, we try to avoid incentives for everyone with open surveys, but rather have a raffle. E.g. instead of 1000 vouchers with the value of $ 1, we raffle 10x $ 50, and we also usually try not to announce it, as not to attract the wrong crowd. It is more to be seen as a "Thank you" rather than a payment for the time. We do not want people to participate for the money (not the right motivation), but of course we want to say thank you.