It's fine in principal really, but have you tried on a maxi dress? It's a beautiful dress but everyone looks different wearing different things, body shape, height, bust, all that to take into consideration and while things look good on a model online or in a catalog or it looks good on the rack, it may not be WOW factor on you. Or it might be. Who knows. Women come in all different shapes and sizes, and dresses are made for a small population of models in a very narrow range of sizes.
Long dresses tend to work more on taller women.
But aside from that, it's not what rules dictate you should or should not do. Take a good friend shopping with you if you're not really sure, and go try some on- not saying you have to buy what you see, but you can't really tell if that mint tribal maxi dress online is 'you' or if you'll 'feel' it the way you think you do in the picture.
I had a pretty clear vision of what styles of wedding dresses I wanted to try on before I went dress shopping. My MOH went with me and insisted I try on some things she picked out. Stubborn as I was, I let her have her wish. And thank goodness for that. The pictures are deceiving. And your mind is deceiving sometimes.
And then consider that you want to find a dress you will be able to wear again more than once.
Also I'm not entirely sure about black shawl at a wedding? When you're going to try dresses on, go look at wraps and coverups, there are a lot of cute bolero style jackets out there that are light enough to not hinder you from getting up and hitting the dance floor, if it's that chilly, and are not going to make you look like the mother of the bride, or simple wraps/shawls too. Try some on, see what feels right. I have a few bolero jackets that I wear when I go out on the town and I don't want a full-on coat to drag around, to go with sleeveless tops/dresses.
Have fun shopping :)
personal experience- I never buy anything online unless I've tried it on in person. I sometimes take a friend if it's something I'm not sure about. I have been to many weddings as a guest so I've shopped a lot for things to wear, and I spent a good amount of time shopping for my own wedding dress and my bridesmaids dresses also to make sure they liked them in person, not just online pictures.