¿Podría usted guardar este ramo un momento mientras termino de pagar en la floristería?

Podría is the conditional form of poder, and here it makes the request more polite and less direct.

• ¿Puede usted...? = Can you...?
• ¿Podría usted...? = Could you...?

In English, Could you...? often sounds softer and more courteous than Can you...?, and the same idea applies here.

Yes, usted could be omitted.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is:

• ¿Podría usted guardar este ramo...?
• ¿Podría guardar este ramo...?

Both are correct. Including usted adds formality or emphasis. Since this is a polite request to a stranger, using usted fits well.

Literally, guardar can mean to keep, to store, to put away, or to save, depending on context. In this sentence, it means something like:

• hold onto
• look after
• keep for a moment

So the speaker is asking someone to take care of the bouquet briefly while they finish paying.

Ramo here means bouquet or bunch of flowers.

In a flower-shop context, un ramo normally refers to a bouquet. By itself, ramo can have other meanings in other contexts, but in this sentence the meaning is clearly floral because of la floristería.

Un momento means for a moment, for a minute, or just a moment.

It is a very common way to refer to a short time:

• Espere un momento. = Wait a moment.
• ¿Puede venir un momento? = Can you come for a moment?

Here it softens the request by making it sound brief and reasonable.

Both are possible, but they are slightly different in nuance.

• mientras pago = while I pay
• mientras termino de pagar = while I finish paying

Using termino de pagar emphasizes that the speaker is already in the process of paying and just needs to complete it.

Also, after terminar, Spanish often uses de + infinitive:

• terminar de comer = to finish eating
• terminar de leer = to finish reading
• terminar de pagar = to finish paying

Because the pattern is:

• terminar de + infinitive

So:

• termino de pagar = I finish paying
• terminó de hablar = he/she finished speaking

This is the normal structure in Spanish. You generally do not say termino pagar.

Because en expresses location, while a usually expresses movement toward a place.

Here the speaker is already at the flower shop and is paying there, so en la floristería means:

• in the flower shop
• at the florist's

Compare:

• Voy a la floristería. = I’m going to the flower shop.
• Estoy en la floristería. = I’m at the flower shop.

Yes, floristería is very common in Spain. In many parts of Latin America, you may also hear florería.

So:

• Spain: floristería
• Many Latin American countries: florería

Both refer to a flower shop, but floristería is especially appropriate for Spanish from Spain.

Because the speaker is referring to a real, current action happening at the same time:

• mientras termino de pagar = while I finish paying

When mientras refers to an actual ongoing or real situation, the indicative is normal.

The subjunctive can appear after mientras when the action is future or not yet realized:

• Espere aquí mientras vuelva. would not be natural.
• More natural future idea: Espere aquí mientras vuelvo.

A clearer contrast is:

• Te llamo mientras voy al trabajo. = real/habitual situation
• Te llamaré mientras vaya al trabajo. = less common, more dependent/future/uncertain style

In your sentence, the speaker is simply describing what they are doing right now, so the indicative is the natural choice.

Yes, Spanish word order is flexible. For example, you could also hear:

• ¿Podría guardar este ramo un momento mientras termino de pagar en la floristería?
• ¿Podría usted guardar este ramo mientras termino de pagar un momento...? — less natural because un momento sounds better closer to guardar
• ¿Podría usted, un momento, guardar este ramo...? — possible, but not the most natural

The original order sounds natural because it keeps the polite request clear and places un momento where it naturally modifies guardar.

Possibly, depending on exactly what the speaker means.

• guardar = keep/hold/look after
• sujetar = hold physically
• cuidar = look after/take care of

If the person just needs someone to physically hold the bouquet for a second, sujetar might be more precise:

• ¿Podría usted sujetar este ramo un momento...?

If the idea is more general, guardar works well and is polite. It suggests keep this with you briefly for me rather than only hold it in your hands.

Este ramo means this bouquet, so it points to a specific bouquet near the speaker.

Spanish demonstratives work much like English:

• este = this
• ese = that
• aquel = that over there

Using este makes sense because the speaker is probably holding or indicating the bouquet they want the other person to take.