You can buy as many Lyft Gift Cards as you wish and load them all to one or more Lyft accounts. They stack.
Lyft Gift Cards never expire.
Do gift card amounts roll over? Yes! If the full $20 isn't used on one ride, the remainder is applied to your next ride.
Can I use a gift card to tip? Just like all Lyft credits, tips are not covered and will always be charged to the credit card associated with your account.
Want more value?
UPDATE: The stars did not show up today (after 24 hours) and I called the Customer Service and was told that Gift Card Purchases do not earn Stars. However, as a one-time exception, they added the Stars for me. The Barista at the Starbucks Store where I purchased the Lyft Gift Card from clearly didn't know what she was talking about then.
And to get the Stars, this might be a lot of trouble to go through for some. So I think it's still easier to just pay for the Lyft Gift Card with Chase Freedom and earn 5% Cash Back.
Now, if you have a Starbucks Account/Card, you can get more value out of this deal.
Simply pay for the whole $20 Lyft Gift Card purchase with your Starbucks Credit/Fund and earn 40 stars. And for Starbucks Gold Members, if you buy this on a double-stars day, you'll get 80 stars!
The value of these stars vary, depending on what you redeem them for. Each redemption costs you 125 stars.
For example, a Venti Frappuccino (one of the most expensive drinks) here in Chicago, the after-tax price is $5.52.
For this, the $/star you get is $5.52 / 125 stars = $0.04416/star.
If you pay the whole $20 Lyft Gift Card with your Starbucks Credit/Fund, you essentially would earn = 40 stars x $0.04416/star = $1.7664 or $3.5328 on a double-stars day.
One way to look at this - in the end you'd get:
$20 Lyft Gift Card + $5 Starbucks Gift Card + $1.7664 - pay $20 = $6.7664 bonus or 33.83% bonus.
or on a Double-Stars Day: $20 Lyft Gift Card + $5 Starbucks Gift Card + $3.5328 - pay $20 = $8.5328 bonus or 42.67% bonus.
Want even more value? Reload your Starbucks Account with discounted Starbucks Cards available online on many sites, or Groupon (which sometimes sell discounted Starbucks Gift Cards), etc before buying the Lyft Gift Card(s).