It sounds like there reason was legit, if they went through the trouble to handwrite you a letter to explain things.
But now the feedback
How do you feel about the whole transaction? I would weigh it out as a whole. Ok, there was poor communication(gave a reason), slow shipping(same reason), and the fact that you did receive your item in time for when you needed it. There are those.
Now, are you happy with what you got? And do you believe what they have told you? Which is really what you should go on. There are things that are beyond someone's control sometimes and I really do try and take that into account if I have had a not so swell transaction like this.
I would probably still leave postive and make mention of the internet problem and then maybe leave a bit lower in stars for communication and shipping time. But the decision is yours and how exactly you feel about it. Hope that helps some, I feel like I am starting to babble