As a full-service shopper, Jen buys groceries for other people and delivers them to customers' doorsteps six days a week near her home in Massachusetts, she told Insider.Talking to Insider about the most common mistakes people make when ordering groceries through Instacart, Jen requested to be referenced by her first name only in order to speak frankly without fear of losing her job, but her identity has been verified and made known to Insider.Sometimes, grocery stores are out of a certain product or type of food.Jen recommends that customers keep their app open while a shopper is fulfilling their order to avoid being disappointed with the groceries they receive. That way, customers can respond to shopper questions about how best to replace out-of-stock items.Customers often get very, very angry with shoppers because we've had to refund an item or maybe we bought the wrong item because Instacart will prompt shoppers to choose a replacement, Jen said.I promise you, I'm not trying to mess up your order. I'm not deliberately trying to buy the wrong loaf of bread, she added.Jen recommends taking advantage of the notes function in the Instacart app to avoid order delays and disappointment.When placing an order, you can click on each item and leave a note specifying your preferences — whether it's bananas that are more yellow-green than yellow-brown, or apples that are a certain color, Jen said.You can also include instructions for delivery at checkout.If you don't want to talk to us, that's fine. Just state in the app: 'Hi, I'm going to be working. Please drop off my groceries and use the side door,' she said.Leaving notes for shoppers makes us more efficient and faster, Jen said.Jen told Insider that a customer once gave her a four-star rating (out of five) because she delivered a box of grapes with a few crushed pieces in the middle. She added that when customers penalize shoppers for issues out of their control — like an out-of-stock item or imperfections in packaged produce — it may affect the amount of money shoppers are able to earn.While a four-star rating may not seem like a big downgrade, when Instacart shoppers receive more than two ratings below five stars out of 100 orders, their average rating drops, Jen continued. Shoppers with lower average ratings can have fewer opportunities to shop for priority grocery orders, which give shoppers the largest payouts and largest customer tips, Jen said.A spokesperson for Instacart told Insider that shoppers with higher average ratings may have access to grocery orders earlier than other shoppers, but a shopper's star rating does not limit them to certain types of orders.Not tipping can lead to a lower quality of service since higher-rated drivers may not be as tempted to pick up your orders, Jen said.Instacart's default tip is 5%, and the app suggests a minimum tip of $2, according to Instacart's website. Customers can change the tip amount before placing an order as well as up to 24 hours after delivery.As contract workers, Instacart shoppers rely heavily on tips, Jen said. She added that she never accepts an order without one.We're providing an honest, hardworking living and deserve to be respected just like anybody else, she said.