Trust me, if you give gift cards on top of cash, you'll have a provider wanting to marry you.  They love any show of affection outside of your cock!  Shower with gifts, you'll be rewarded dude!

Say you start with a $50 gift card from Target/WalMart/Lowes, etc.  
Then you "reload" it by adding funds (10 here, 20 there) at other times until you have enough.
Simple enough to check/keep track of the amount on the card online.

Not sure how the provider will react though.
Most of the time when I give a gift card, I also give the recipient so there's proof of purchase - just in case the card is demagnetized, for example.

You might find it easier to stash funds here and there then have a bank make larger bills if needed - unless your wife knows all of your stashes.

Heck they frequently deed to count the cash in front of you and several report needing to carry markers testing for counterfeits.  

I see these issues
1).   Proof of available funds - what if you just lifted it from the grocery store?  What if you already drained the funds
2). Valid where she wants to spend - I mean she may not shop at applebees and target
3). Convienence - she can use cash anywhere.

Save the gift cards for the thank you,  ladies love that extra appreciation.  But pay for the session in cash.