I have had people fill in my words but I don’t believe I’ve ever been perceived as being rude because of it. In a sense you are a bit fortunate because “thank you” is easily something you can practice because there are so many opportunities to say it. I would suggest using your thankfulness first whatever they did as positive energy to help you say the words “thank you”. You can even smile while saying thank you, then if you start stuttering, at least the person will feel at ease by you smiling