I hear you ladies! It's so easy to buy these beautiful dresses!
Here's the thing with the Halter straps, the front straps are slightly angled ~ those should be corrected first. Some do and some don't, it's really a matter of preference. However, it will cause a slight gap when you sew the straps to the back. If you want your straps to lay flat and in line with your bra, you need to remove the straps, put the dress on wearing the bra you plan on using with said dress, lay the straps on the bra straps and pin them in. It is a two person job unless you have a dress form.