Take note of how quickly time flies by. If you find that you're doing tons of dull, repetitive work in a week, it's time to Hire Ecommerce Operations Services.
Make sure to keep an eye on your mistake rate. A knowledgeable agency would be able to quickly reduce the percentage of fulfillment mistakes if it is over 2% if it is happening.
Be sure to consider the growing stage also. Most of the advantages of outsourcing the operations go toward the brands which generate annual sales worth of $500k to $5M. If it's below, then the result might not be worth what it costs.