If you want to keep your AWS usage within the free tier, you need to stick to t1.micro instance type. Juju defaults to a larger instance type by default to more easily support multiple, large Jujus models.
instance-type constraint will instruct Juju to use a smaller instance type than its default for its controller (via
--bootstrap-constraints) and any models (via
juju bootstrap aws aws \ --bootstrap-constraints 'instance-type=t2.micro' \ --constraints 'instance-type=t2.micro'`
If you would like to learn more about which constraints that Juju supports, we have thorough documentation.