For example, You can charge everyone a price say $9 but for this customer you want to charge them only $8, the only way to do it is by coupon or by edit the package to override the price every time they order it.
Maybe they haven't installed the pecl edition of the extension. They need to do the Pecl modules on WHM. Or it's best to move to you own environment because then you can do itself and get it working. With InterWorx you can install it with yum commands, but if you do that on a cPanel server it can break, so they have to do it via WHM.
I don't believe it's Blesta, you've done everything correct, and I've done piping many of times on cPanel & InterWorx, and never had an issue except that once helping one client which piping and the crons didn't work. It confused us both and he said he also hated cPanel.
Ok on Forwards under Mail, click that, and then go to Advanced, and then Pipe to program.
Paste the command from Blesta's support department under piping.
Then save it.
Weird it might be the like the issue a client had, but he's crons didn't work on cPanel either which is why he moved to another control panel. Are you hosting on your own VPS or a reseller account?