What should I do? While putting her toys away she found the letter on a toy shelf.
Have you told her about how you feel?
I'm talking about my feelings to her.  But now I'm not sure if I can put those toys away by myself, they're too heavy
You might be feeling too tired right now, so you should rest.  But in the future, it is better to clean up after playing. Otherwise you will have a hard time finding your toys among the mess, and will have a much bigger job to deal with later.
That sounds like a good rule.  For now, I'm feeling too lazy.
It's good to clean up toys that you are no longer using,  before moving on to other toys. This will help you to avoid having a huge mess at the end of the night.