I ordered the Big-Y on top of my yDNA-111 STR test and it specifies many SNPs now that Big-Y is complete, but FTDNA continues to offer to sell me additional SNP tests and packets, in particular the I1 - Z60 SNP Pack.
Does it make any sense to order these? Will they provide additional information not in the Big-Y results? If so, why are some SNPs included in the Big-Y but not others that I may have?
Before I ordered any extra SNP tests, I would wait a few weeks or months. Big Y tests an awful lot of markers, and unlike most other tests, it also discovers hitherto unknown markers (kind of like a "full system scan" on your computer, if that's a proper analogy). Whoever is studying your results needs some time to analyze what has come in.
Also, ask Family Tree to send you the .BAM file from your Big Y test, and consider submitting the results to another testing company such as YFull, which can further analyze and refine the existing data from that file.
My recommendation is to join a genetic project for the haplogroup to which you belong. The project administrator(s) can help you decide whether to test further at this point or not.
Understand that since the Big Y was created, other SNP markers have been discovered, so depending on what your actual results are, there may be new markers that are worth testing. (This is very true of my own Y-Haplogroup.)
As a way of assisting you further, may I ask what Big Y determined your haplogroup classification to be?
In general I would say no, but there are couple of reasons in which you might.
I covered them in this answer "If you plan on getting a BigY is there any value in going beyond Y37 to a degree too.
The primary one is that the BigY test is a discovery test looking to detect ANY SNPs.
The problem with this discovery test is that it misses SNPs. That is why a BigY test done by two brothers can produce vastly different data file sizes on 400 megs and 600 megs respectively. They technically should have all of the same or most of the same SNPs and be very close in size of their dataset.
A part of this difference, and missing things, is also resolution of the test. BigY the higher the resolution the more SNPs you are likely to find. BigY is considered High but FullGenomes does offer a higher resolution test.
That said I have had BigY results range from 380 megs to 970 megs.. the later having more than 70 newly discovered SNPs never seen before. Their placement in the tree for FamilyTreeDNA took several months and my persistence after YFull analysis placed them appropriately and revised their location a time or two.
SNP Packs are prescribed (vs. discovery tests) that are programmed to look for the presence of specific SNPs. That is all, it won't discover anything new and it will retest SNPs found or not found in your BigY result set regardless. That is why they say make sure you order the right pack.. looking at some of my BigY results I manage some of the SNP packs would only test about 6 additional SNPs.. which is still cheaper than ordering them individually.
I have only ordered an SNP pack once after a BigY as a verification of a result..and had no different results. SNP packs do not go as far down the tree as a BigY a lot of the time when ordering them by themselves from ordering them on individuals without BigY.
SNP packs do get updated about once or twice a year, and my decision to order one after a BigY would be based on how many layers lower in ftDNA's tree will it take me if at all beyond what is that information already on YFull or if I am questioning the results against another one.
I cannot emphasize enough the value of transferring your results to YFull, as odds are you will get several layers deeper into the Ytree than FamilyTreeDNA goes as well as provide useful matching criteria vs. anyone in your top level haplogroup that FamilyTreeDNA provides you that has taken the BigY.
