Guild Wars 2

Increases the stack size by 250. Does this mean you can send items via mail by bulks of 500 per slot?

It only increase the BANK stack size. Not the inventory, neither the mail.

Read the patchnotes:

The Collection Expander is now available in the Gem Store in the Upgrades category for 800 gems each. Each Collection Expander
-increases
-the stack size
-allowed for all collections
-by 250
-to a maximum of 1000.

MauricioCezar.2673:

It only increase the BANK stack size. Not the inventory, neither the mail.

Actually, only the stack size in the bank’s collection tab. The stack sizes in inventory tab remain unchanged (as well as stack sizes everywhere else in the game).