I prefer

Visa or AMEX gift cards. Easy to find and the lady can spend them anywhere
Hugs and kisses
TL

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.