OK, that's quite a significant change of product type then!
With bitters you'll probably have an ABV that makes life easy in terms of microbiological concerns.
Some types of spirits will go through a cold filtration stage as part of processing, to reduce/prevent flocculation. Hopefully this will have already been considered as part of the brand owner's recipe/process development.
If you're using water in the recipe then this can sometimes create a challenge, as different water sources will have different compositional characteristics - changing this can in some cases lead to formation of certain precipitates, development of hazes etc. I've seen quite a few alcoholic drinks producers caught out by this one (To be fair it can also affect soft drinks producers too, and they're equally indifferent to it until it hits their product...).
If the customer is supplying what is effectively a finished blended product that you need to literally just put into bottles then your life should theoretically be fairly simple. Nonetheless, I'd ask them to define exactly what process they're expecting you to undertake, so you can review and confirm that you really can do what is expected of you. (I've seen a few "oh, we'd assumed that you'd know you were supposed to do x to it" incidents with brand owners / contract packers over the years),
If you're being given a set of component parts and are expected to assemble the recipe, particularly if using your own potable water source(s), then I'd strongly recommend trialling it in the lab first, and again I'd be asking exactly what process the client is expecting of you.
It could turn out to be a product that requires no special processing etc., but best to be absolutely sure of such things beforehand.
Obviously you'll also need to do the usual food safety bits in terms HACCP / HARPC or whatever is applicable in the US and/or required for your certification, possibly check whether your customer wants you to add this to your certification scope etc, and also look at glass/brittle materials control if you're not already using glass (I'm assuming you'll be packing into glass bottles) on site.
N.B. I have no idea how the duty/tax system works for alcohol production in the US, so you may also need to look into that. Over here you'd need to register with HMRC (tax collection authority) to store and/or process this type of product, or have it supplied to you with the duty already paid.