The supported notification protocols for SNS are HTTP, HTTPS, Amazon SQS, Email, Short Message Service (SMS), and AWS Lambda.
When a user is subscribed to a SNS topic, a message is sent to the endpoint (email, SQS, HTTPS/URL, Lambda or SMS) and the endpoint has 3 days to confirm the subscription. Until a subscription is confirmed SNS notifications will not be sent to that endpoint.
Messages can be sent in JSON, or RAW format, payload can be upto 256kb. You are charged by 64kb so a large 256kb message will be charged as 4 SNS messages.