Redbubble, Spreadshirt, Merch by Amazon or Zazzle are good services. They take care for customer support too, the issue with them is that you have not a control over the quality of the products.

You can control the quality of your designs but that is all. If you want to build a POD business, then you need go to Printful or Printify to print and sent your products.

While you can control the quality here, you will need to handle the customer queries.