色々と検討したけど、Fn::Joinで実現するのが最短そう。

custom:
  apiEndpoint:
    Fn::Join:
      - ""
      - - "https://"
        - Ref: "ApiGatewayRestApi"
        - ".execute-api."
        - ${self:provider.region}.
        - Ref: "AWS::URLSuffix"
        - "/"
        - ${self:provider.stage}

呼び出すときはこれ。

postExample:
  handler: src/functions/example/post.handler
  environment:
    END_POINT: ${self:custom.apiEndpoint}
  events:
    - http:
        path: /example/
        method: post
import axios from 'axios'

export async function handler(event, context, callback) {
  const result = await axios.post(process.env.END_POINT + "/example")
  callback(null, {
    statusCode: 200,
    body: JSON.stringify({
      result
    }),
    headers: {
      'Access-Control-Allow-Origin': '*',
      'Access-Control-Allow-Credentials': 'true'
    },
  })
}