Waiting for forEach loop in aws lambda function

You're already using async, so use map to return a list of promises, and await them all with Promise.all():

module.exports.shipments = async (event) => {
    const axios = require("axios");
    let data = JSON.parse(event.body);
    let url = data.apiURL + "/api/1.1/wf/bulkshipments/initialize";
    let patchURL = data.apiURL + "/api/1.1/obj/company/" + data.companyID;
    
    let promises = data.shipments.map(item =>
        axios.post(url, {
            batchID: data.batchID,
            companyID: data.companyID,
            shipment: item})
        })
    );

    await Promise.all(promises);

    return {
      statusCode: 200,
      body: JSON.stringify({
        message: 'Created successfully!',
        totalShipments: data.shipments.length,
      }, null, 2),
    };
};

To have each call wait, instead of firing all the post requests at the same time, use a for...of loop:

module.exports.shipments = async (event) => {
    const axios = require("axios");
    let data = JSON.parse(event.body);
    let url = data.apiURL + "/api/1.1/wf/bulkshipments/initialize";
    let patchURL = data.apiURL + "/api/1.1/obj/company/" + data.companyID;

    for (let item of data.shipments) {
        await axios.post(url, {
            batchID: data.batchID,
            companyID: data.companyID,
            shipment: item})
        });
    }

    return {
      statusCode: 200,
      body: JSON.stringify({
        message: 'Created successfully!',
        totalShipments: data.shipments.length,
      }, null, 2),
    };
};