If you can’t give us anything this time, but would like to do so in future, please just hang on to the unused bag and pop it into the new one (along with your pre-loved items) next time we visit. You’ll usually only have to wait 5 or 6 weeks.
If you’re not planning to donate at all, you can recycle the bag or use it as a bin liner, but please remember to turn it inside out, so as not to confuse bin collectors.