For a good online sales proposal software, I would recommend Clientsky (https://www.clientsky.com). They are great for small-medium sized businesses who want a very professional looking proposal that their clients can accept online. ClientSky has templating features that speed up the proposal development process (https://www.clientsky.com/howitworks) and it also has the ability to integrate video/images/examples/etc. For your business, it might be really nice to show some examples of your work in the proposal or even include a video. All ClientSky proposals are also mobile responsive so they look great on tablets and smartphones. Check ClientSky out and let me know if you have any questions. We use it and love it.
I think they have a 30 day full featured free trial too, which is cool to get started.