If you are using AWS for testing purposes, look for immediate charges that are taken by aws. If you ignore them, they might be intolerable for your credit card (As debit cards are not supported by them yet!). Kindly look forward some points that are charging to credit card (As I will keep mentioning from my own experiences).

• Don’t use elastic IP if you are not planning to map your domain. Elastic IP is free for running instance only. If you stop a running instance during any testing, and your elastic IP is not disassociated, then it starts per hour charges. Example is mentioned in screenshot here. 

 

 

• Elastic IP can be free for one linux and one windows instance, as they both are together free in FREE TIER plan as on august 2018. But third IP should charge. But again, if you stop either of running instance, don’t forget to first disassociate IP and then release IP before stopping instance.
• For domain mapping, you can directly use Route 53 service, even while you don’t have elastic IP. As your instance is stopped, and started again, you will have to add A name record for newly allocated IP, and map the same in your domain registrar’s records. That saves your elastic IP charges. Elastic IP should be used only if you are sure that your instance is going to be permanently mapped to a domain. In that case, you can save Route 53 charges, by just using elastic IP and getting your domain permanently redirected to only elastic IP, no need to update other records like C name and DNS records in that case. That will save Route 53 charges. Now it’s a matter of practice that, which one is costlier in monthly usage, that I am not sure. If you have that judgement, please post in the comment below.
• Don’t use another linux instance while one is already running. Similarly, Don’t use another windows instance while one is already running. It’s clearly mentioned in Free Tier terms that, both in single number are free. Another one will start charging per hourly as “on demand instance”.

 

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• This screenshot explains that another linux instance in same month levied charges as “on demand” instance. 
• Another charges to look for is that, if you have tested more than one domain in route 53, it charges flat 0.5 USD per hosted zone, so if you have tested any domain, and then disconnected them from instance, and you have forgotten to delete hosted zone from route 53, it will keep charging as mentioned. So as soon as you keep your domain fully tested, either keep it running, or wipe up every single thing before checkout, as mentioned above, e.g. Hosted zone from route 53, any elastic IP, any security group, any detached volume stored for instance, or instance itself should be terminated after stopping. 
• Although there is option for billing alert, it’s for total amount you define to alert. I request amazon guys to setup customized alert for all different services, like alert for all segments chargeable, so it will give real time idea about financial planning. Also, there can be sms alert system together with email, which is far better while you are offline. 
• All inbound traffic is free, but some regions outbound traffic is charged. This is absolutely out of control for any guy, to monitor outbound traffic 24*7. Moreover, we have to believe them, as we are humans and not servers. So I have not yet found way to manage this type of charges. But this question comes, when you are that much popular eh!