Seems your question might be in tow parts
In regards to setting your shipping price, there really is absolutely no way of knowing without the supplier telling you. I find their attitude quite bizarre and I would like to pass this onto admin for a follow up with them regarding this. Just let me know who they are and we'll see what on earth the issue is with them.
The second part of your question seems to focus on price. If you are finding competitors that are selling what you are trying to also sell, but at a fraction of your sale price, then that's certainly an issue. How much research have you actually done on the market so far?