If this was mett;ler or Gutterman thread it's an easy mistake to make, to pick up the wrong weight. The spools are quite similar. As it was an error on their part, just call or email them and I'm certain they'll send another spool immediately, probably without taking the old one back. It almost costs more for them to pay shipping it back again than it's worth.
No biggie, just an inconvenience.
Jan in VA