"Msg & data rates may apply" is a notice that depending on your customer's cellphone plan, their carrier may charge them standard message rates and data charges to receive your text messages or reply to them.
Almost all cellphone plans come with unlimited text messaging included and buckets of data, so your customers are very unlikely to incur any costs.
If they have a pay-per-text plan, then whatever the cost to send or receive texts is, is the rate that may apply.
In any circumstance, there is no extra cost for your customers to receive text messages from SimpleTexting.
If you are looking for more details on all things related to the cost of texting your customers, look no further. Here are some resources to help get you started.