First of all, regarding credits, - yes, if you have promotional credits for EC2, these credits do cover any Elastic IPs for which you are charged, because EIPs are part of EC2 service: https://aws.amazon.com/ec2/pricing/on-demand/
However, please note that you are NOT charged depending on your quotas. You are charged depending on the usage. Also, you will not be able to create any additional EIPs above the quota. For that you would have to increase your quota.
With Elastic IPs - you are NOT charged for it if EIP is attached to a resource (e.g. EC2 instance, network interface). But if you don't use your EIPs, and it remains unattached (you can see the status in the console or using API) - this is when you will pay a fee for the Elastic IPs that you allocate to your account. You can check more info on the pricing page: https://aws.amazon.com/ec2/pricing/on-demand/
You may be able to request the increase of your account quotas for particular service or feature, but this does not impact your billing. What does impact the billing is your actual usage. So, for example... let's say if you have a limit of 5 EIPs. And let's say you already created 5 EIPs. As long as all 5 EIPs are attached to a resource - you are not charged for them. If you request to increase that quota, let's say, to 10 EIPs, and then you create extra 5 EIPs and keep them unused (not attached to any resource) - then you will be charged a fee for each unattached EIP.
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