You’ve been lucky then, I use MSL a lot and if the value is over the £135 then I’ve always received the VAT bill from FedEx after delivery has taken place. Usually the bill comes first by txt message, then by email and eventually by snail mail.
It’s been mentioned a few times in this thread but worth repeating. You don’t need to wait for the letter to arrive from ParcelForce. Once you can see on the tracking that the parcel is in the local depot you can ring ParcelForce who will translate the tracking number into the payment number allowing you to pay online (and usefully pick a delivery date that suits you). You were unlucky to get charged on a €100 item, that doesn’t normally happen although as this thread constantly proves the rules aren’t always applied correctly!