Definite articles are not generally necessary with plurals. Where a singular, countable noun is used, you generally need an article or determiner; if it is not specific, it will usually be an indefinite article. Where it is plural, you don't need an article or determiner, but can use one where it is appropriate. A definite article is particularly dubious in this case because it's only a case that they may be needed.
You can have a word there, if you want - you can use any, which fits well with the may and the fact that the nature of the services are unknown at the time of writing (as they will vary between customers). However, the actual meaning of any may be assumed without actually writing it. Whether you include it or not is largely a matter of style.