Even if the person perceived it as yelling, their response was inappropriate. When dealing with consumers, there are no feeling words or responses. The supplier of services or goods must provide answers and ask questions simply. Anything else causes a diversion and perhaps the loss of a customer.
If the bold or italic function isn't available, you *may* try asterisks for emphasis. You could also try limiting your point to three sentences or less and using what feels like an overabundance of paragraph breaks.
It would seem most people are easily confused and require some coddling. Pretend everyone has had a really bad day and proceed from there.