For 20 years I have been a parent of a child with special needs. You can imagine how often I have shopped for, and with, my child. Over the course of those years, I have encountered numerous vendors, proprietors and salespeople. There are retailers to whom I would never return and those who I frequent. There are common characteristics of those vendors, and quality of merchandise is not high on the list.
Foremost, both the sales personnel and the shop are inviting. When I walk into a store I want to feel welcomed, and my daughter, who has Down syndrome, wants the same. There is often an assumption that a child or an adult with a disability is not aware of, or cares about, common courtesy. A simple "Hello, how are you today?" is all it takes to feel welcome. An acknowledgement of our presence goes a long way.
It is a good practice to assume that the child or adult with special needs understands all or most of what you say. Oftentimes a person's expressive ability (speaking) is much slower or more impaired than his/her receptive language abilities. This means that although a child may not be able to answer your question, he/she does understand the questions and comments you are making. To this point, it is imperative that you develop the practice of talking to the child, not about the child. There is nothing more offensive than to have my daughter standing next to me and a person ask me about her, as if she is not there.
In addition to speaking directly to the child, it is important that you give the child time to process your questions. Don't make the assumption that because she or he did not answer you, he/she is being rude. My daughter was once asked a question by a vendor, and over 2 minutes later walked back over to him and gave an answer. From an outsider point of view, it looked as though my daughter, 1) didn't understand the question, and 2) was inconsiderate. Neither was true, but because the vendor quickly gave her a positive response, my daughter was encouraged to engage more. As a result, she seeks out that vendor and we are in his shop frequently.
Just recently, my daughter and I were shopping for gardening supplies. When I came home, I told my husband that we always need to buy our flowers and vegetables from this vendor. Here is why: As my daughter and I walked around the shop, the saleswoman nodded her head "hello" (nonverbal communication). When she was finished with her current customer, she came over to us and introduced herself, shaking my daughter's hand (interaction). She could see we had already started to fill our basket with items and she turned to my daughter and said, "Oh, I see you've already made some great choices! I love the succulents that you have picked out!" (engagement and positive reinforcement). She then asked us if we needed help and I said yes, we were looking for a plant to fill a planter. She turned to my daughter and asked her if she wanted a flower for the new pot. She waited for a response. When my daughter said "yes," she then said to her, "Would you like a red flower?" (Waited for answer, which was "no.") "Would you like a yellow flower?" (Waited.) "No." "Would you like an orange flower?" "Yes!" Immediately she responded, "I love orange, too! Let's go find an orange flower for you to plant."
As the proprietor of the store rang up those items, I asked her about the saleswoman, whose name was Pat. Was she a special ed teacher or therapist working here for the summer? No, Pat is a master gardener. Is she a parent or sibling of a child with special needs? No, Pat is just Pat.
Pat had no "official" training or education in working with children with special needs. She was simply enthusiastic and engaging with both myself and my daughter. Her interactions immediately built a rapport and she gave my daughter what we all seek the most: dignity and respect.
One final thought: Be aware that anyone in your store may have a family member with a disability or special needs. How you talk about people will be noted. When you use "person first" language ("I have a client who has Down syndrome," rather than "that Down syndrome girl"), family members take note. When you interact directly with a person with a disability, family members take note.
I once took my children to a new salon for haircuts. Afterward, my son, who was only 10 at the time, pulled me aside and said he always wanted to get his hair cut there. I said I didn't know he cared about his hair that much, and he said, "It's not the haircut. They were nice to Emily." He has been getting his hair cut at that salon for the past eight years. We all have.