it obviously depends on alot fo things. How you obtain your figures, whos buying the album. Do you have proof of sales.
What sorta label/ artist you are. Where you have tried to previously sell your mixtape. What you have planned for the future.

It doesnt come simpyl as, oh I made a mixtape and sold 1000 copy's they will sign me.

It may pay to have a contact at a label/distribution company aswell, so you have someone to adresse what your sending to.
Otherwise if it simply your music in a envolope, they will most likley trash it instantly. on the simple fact that they get 1000's of albums sent to them a day.