Birthday invitation, free
A card you can send in a minute, with a yes or no on it so you know how many people to expect.
Say the finish time if there is one
Most Indian invitations say "7 pm onwards" and leave the rest open, which is right for a house party and wrong for a booked venue with a slot. If you are paying by the hour, say so on the card.
One line does more than a theme
What people actually want to know is whether to bring something, whether to eat first, and whether children are invited. One sentence covers all three and saves you twenty messages.
Sawaal
Can I use this for a surprise party?
Yes, and the private link is the reason it works - the page is not listed anywhere, so it is only seen by the people you send it to. Just remember that anyone who has the link can forward it on.