If I have a very close STR Marker match, does that mean we are closely related?

It may, but not necessarily. Persons with the same terminal SNP are more closely related than those with a different terminal SNP, even if you have a close STR marker match to someone with a different terminal SNP.

Why should I consider a Big-Y Test?

The Big-Y700 test for a R-L226 man will check all the terminal SNPs for the Cluster AND provide you with a list of 'private' SNPs, those that have occurred in your ancestral line since your line left the R-L226 Tree.   As others test Big-Y700, you may find that another person may share one or several of your 'private' SNPs, and so define a further branch in the tree.

Should I order the R1b - L226 SNP Pack to learn more about my terminal SNP?

If you have already taken the Big-Y700 test, then NO, all the SNPs in the R1b-L226 SNP Pack have been tested for you in Big-Y700.

If you have NOT tested Big-Y700 then the R1b - L226 SNP Pack may help discover your branch, however it is now very outdated, and so Big-Y700 is still the best option to find your terminal SNP.  The R1b-L226 SNP pack will not reveal your 'private' SNPs.