Because the N54 is direct injection carbon build up is inevitable. I think it is fair to consider valve cleaning a standard maintenance item with this engine. Unfortunately chemical cleaning is not going to do much for your valves at 88k. A physical cleaning (walnut shell blasting or wire brush scrubbing) is the only way to remove the carbon build up. I am not familair with the BG products but from what I found when searching they are chemical products. At 88k I'd be really surprised that a chemical cleaning would remove the carbon build up. There is a member in Coral Springs that is doing walnut shell blasting for $300... http://www.e90post.com/forums/showthread.php?t=769892
Also $100 for an oil change seems a bit steep. With a coupon the dealer here in Tampa (Reeves) does it for $89.95. If DIY and you purchase the filter and oil from the dealer it is around $60.