you can go to the post office and ask for tyvek envelopes- they will give them to you- they are sturdier-
the problem with just using manilla envelopes is they tear really easy- and the postal workers are not really known for their careful handling of packages. the tyvek envelopes may actually weigh less too.
or padded envelopes are also good to use for sending (squishy's)
the ones you have really are not (built) for squishy- they are better saved for documents...flat