Putting the donation in plain view (in an envelope, of course) at the start is definitely the best way. Having said that, a lot of top-end SP's don't even notice right away - but believe me, it's best to simply avoid the awkwardness of any SP having to ask for the $. Kills the mood you're trying to build. I've had 4-5 SP's who didn't realize I put the donation at the start, so if I go to tip them, they always look at me funny (like "hmm, that seems awful light" - LOL) - I've learned to recognize the look, and pointed out the donation left at the start. I'm probably going to remember to say "here's a tip with the donation I left at the table" from now on, LOL. To be honest though, except for the 5 secs of awkward looks, when a SP doesn't realize I left the donation at the start, it actually increases my opinion of them, since they've kind of committed to the full atmosphere creation, and money later philosophy (but always get paid, of course). But either way, best to be proactive and leave the envelope on the dresser/table where's it's pretty visible.
Back to OP - 1st shower is always on your time - some of the higher end agencies, once they know you, if the SP is ready, you sometimes get the call 5 mins early, which is nice. Either way, it's only 4-5 mins lost at most, I mean, no one is going there looking like a pig, you only need a body wash and mouthwash gargle (which I do before I head up anyways, but SP comfort goes way up with a great clean first kiss IMO), this only should take 4-5 mins easy even drying off (I mean, you're not there washing your hair and shaving, or lingering lol). General rule of thumb, I mentally see these as 45 min visits - 5 mins at start, and 10 mins at end (which allows me just enough time to take a 2nd shower, and leave 5 mins b4 app't time, which is definitely appreciated). Other rule of thumb with incall - ALWAYS take a shower if asked at the start, even if you've showered 30 mins or less before getting there - there's no way the SP knows that, and really, unlike outcall, you've had to go some time after showering, they won't know if you're really clean or not, and so showering at the start of an incall no matter if you did just before coming to the incall, that tells them you respect their wish to be comfortable and clean. And while it might seem like a bad deal (lose 10+ mins of your hour, 5 at start, 5+ at end), in reality, doing this and being cool about it - it shows respect to the SP, and that's not missed IMO, and goes a long way to starting off with a great atmosphere.