Case insensitive verification of HMAC / Base64 signature
Your worst case scenario is that all $18$ characters in the Base64 string are letters — this allows for $2^{18}$ possible collisions due to case-insensitivity. A normal SHA-1 hash is $160$ bits, and therefore has $2^{160}$ possible combinations. Divide by the number of collisions, and you have an effective strength of $142$ bits.

That said, I wouldn't necessarily recommend this. Far easier and less dubious would be to use Base32, which only uses uppercase letters.