If we make an analysis of the sentence, we will see it divided as: [私-は(subjective)] // 買物-が(objective) // したい (verb)// ので(す) // が. *のです(nodesu) - that ん/の is a particle linked to the predicative [objective+verb]. This, when added to the copula verb です (polite form), makes a STATIVE construction. Roughly translated, the statement would sound like: "The fact is/the reason is that (の) I would like to do some shopping. (買物がしたい) + です". *が: puts a great emphasis on the stative form above. It is usually used when you are waiting or expecting some kind of answer, as: "The reason is that I would like to do some shopping, so... (could you please help me?)" or VERY roughly "I happen to be wanting to do shopping..." [Remember that it is mainly used in spoken language. i.e. dialogues.] A common use of the が-construction, is when you are on the phone, either calling or answering. ex. Ække Nækkepennですが(polite) = ですけど(colloquial) , Greensilenceいらっしあいますか? - "It is Aekke Nækkepenn speaking, is Greesilence home?" *In conclusion: the mysterious が-particle (the one you put at the end of the statements, be careful)can be omitted in many cases. however, it adds a lot of emphasis if it is used when you are waiting for some kind of reply from those you are speaking to. one more ex: 肉が食べたいんだすが. (niku ga tabetai-ndesu ga.) "I would like to eat some meat... (do you have it?)". The reason of those construction, is that asking or requesting something too directly may sound too inpolite to ask very directly, so you ask for an answer without asking for it. =P

If you ask someone to make time for shopping "kaimono ga shitainodesu ga dekimasuka?" ≒Could i shopping something. Could I ? "kaimono ga shitainodesu ga" is midfrow in sentence you just feel want to shopping "kaimono ga shitai" ≒I want to go to shopping

aspiration_criteria, thank you for the compliment, I really appreciate that: as a "philologist", knowing grammar is very important. I wouldn´t say I´m either a native or a non-native Japanese speaker, since half of my family is from there. But, as you can notice from the map on my profile, I never had the chance to live there long enough to get in touch with many "natural expressions". So, if you notice that some of my examples sound weird or unnatural, please add a better example to it. Thank you in advance

aekke_naekkepenn knows and explains the Japanese grammar pretty well. Sorry that I'm not a specialist of teaching Japanese, which means that I'm not eloquent about the strict Japanese grammar. Here in italki, I saw many non-native Japanese speakers explain the Japanese grammar in detail better than most Japanese to the level where I myself learn it and how to explain it in English to others... However, in my opinion, native speakers have advantages in that they know fluent and natural expressions in their languages without lots of explanations. I'd like to help you in that aspect of Japanese.