Alright i found this out on accident
so then
1. you click on one of the attachments
2. buy the attachment
3. when it says do you want to attach it? don't click attach
4. then click attach and click the item you want quickly and you will attach it to your weapon
if someone else found it then sorry X_X and did it first X_X
btw you can't take it off lol so yeah. it would say you have it on but you the crappy one too lol.
so its like you buy a first supressor and then you click the itech quickly and get that lol. its awesome =D